The original word base of Esperanto contained around 900 root words and was defined in Unua Libro ("First Book"), published by L. L. Zamenhof in 1887. In 1894, Zamenhof published the first Esperanto dictionary, Universala vortaro ("International Dictionary"), which was written in five languages and supplied a larger set of root words, adding 1740 new words.

The rules of the Esperanto language allow speakers to borrow words as needed, recommending only that they look for the most international words, and that they borrow one basic word and derive others from it, rather than borrowing many words with related meanings. Since then, many words have been borrowed from other languages, primarily those of Western Europe. In recent decades, most of the new borrowings or coinages have been technical or scientific terms; terms in everyday use are more likely to be derived from existing words (for example [a computer], from [to compute]), or extending them to cover new meanings (for example [a mouse], now also signifies a computer input device, as in English). There are frequent debates among Esperanto speakers about whether a particular borrowing is justified, or whether the need can be met by derivation or extending the meaning of existing words.

Origins

Esperanto occupies a middle ground between "naturalistic" constructed languages such as Interlingua, which take words en masse from their source languages with little internal derivation, and a priori conlangs such as Solresol, in which the words have no historical connection to other languages. In Esperanto, root words are borrowed and retain much of the form of their source language, whether the phonetic form ( from ex-) or orthographic form ( from team). However, each root can then form dozens of derivations that may bear little resemblance to equivalent words in the source languages, such as (government), which is derived from the Latinate root reg (to rule).

Word formation One of the ways Zamenhof made Esperanto easier to learn than the European languages predominant at the time was by creating a regular and highly productive derivational morphology. Through the judicious use of lexical affixes (prefixes and suffixes), the core vocabulary needed for communication was greatly reduced, making Esperanto a more agglutinative language than most European languages. It has been estimated that on average one root in Esperanto is the communicative equivalent of ten words in English.

However, a contrary tendency is apparent in cultured and Greco-Latin technical vocabulary, which most Europeans see as "international" and therefore take into Esperanto en masse, despite the fact they are not truly universal. Many Asians consider this to be an onerous and unnecessary burden on the memory, when it is so easy to derive equivalent words internally (for example by calquing them, which is what Chinese often does). This sparks frequent debates as to whether a particular root is justified, and sometimes results in duplicates of native and borrowed vocabulary. An example is "calligraphy", which occurs both as a calqued ("writing of beauty") and as the direct borrowing . A similar development has also occurred in English (brotherly vs. fraternal), German ( vs. for ornithology), Japanese ( vs. for baseball), Spanish ( vs. for basketball), French ( vs. ), and other languages. However, although the debates in ethnic languages are motivated by nationalism or issues of cultural identity, in Esperanto the debates are largely motivated by differing views on how to make the language practical and accessible.

Affixes One of the most immediately useful derivational affixes for the beginner is the prefix , which derives antonyms: (heavy), (light); (upwards), (downwards); (to love), (to hate); (light), (darkness). However, except in jokes, this prefix is not used when an antonym exists in the basic vocabulary: (south), not "malnorda" from 'north'; (to be lacking, intr.), not "malesti" from 'to be'.

The creation of new words through the use of grammatical (i.e. inflectional) suffixes, such as (mere) from (only), (contemporary) from (then), or (sight) from (to see), is covered in the article on Esperanto grammar. What follows is a list of what are usually called "affixes". Most of them, however, are actually lexical roots, in that they can be used as independent words and their relative order in a compound is determined by semantics, not grammar. They are called "affixes" mainly because they derive from affixes in Esperanto's source languages. Some are true affixes in that, although they may be used independently, their order within a word is fixed by the grammar. Only a few cannot be used independently and so correspond to how a typical affix behaves in English.

When a root receives more than one affix, their order matters, because affixes modify the entire stem they are attached to. That is, the outer ones modify the inner ones. Most affixes are themselves roots, and as such have an inherent part of speech. This is indicated by the final part-of-speech vowel in the suffix list below. A few affixes do not affect the part of speech of the root; for the suffixes listed in the tables below, this is indicated by a hyphen in place of the final vowel.

List of lexical suffixes pejorative (expresses negative affect or a poor opinion of the object or action) (to scrawl, from 'write'); (foul weather); (a hovel, from 'house'); (to gape at, from 'look at'); (tawdry, from 'beautiful'); (junk, from ); (to screw up, with ); (yuck!) frequent, repeated, or continual action (often imperfective); as a noun, an action or process (to keep on running); (a speech, from 'talk, speak'); (to carry on); (continual) a concrete manifestation; (with a noun root) a product (food, from 'eat'); (news, a novelty, from 'new'); (an ice[cream], from 'ice'); (beef, from 'bovine'); (a snafu, from and ); (a thing); a member, follower, participant, inhabitant (a Christian); (a US American) [cf. (a continental American)]; (a crew member); (a classmate, from 'same' and 'course'); (a kindred spirit, from 'same' and 'idea'); (a member) a collective group without specific number (a forest, from 'tree'); (a dictionary, from 'word' [a set expression]); (humanity, from 'human' [a set expression; 'crowd, mob' is ]); (a flock of sheep); (a fleet of ships); (a society [group of members], from ); (a government, from 'rule, govern' and ); (a herd, group, set) masculine affectionate form; the root is truncated (Jack); (daddy); (bro); (dear friend); (the 'boys') possible (believable); (visible); (possibly) an abstract quality (friendship); or (goodness); (Italianesque); (character [sum of qualities], with ) augmentative; sometimes pejorative connotations when used with people (a mansion, from 'house'); (a giant, from 'man'); (a tome, from 'book'); (boiling hot); (to guffaw, from 'laugh'); (great, humongous) a place characterized by the root (not used for toponyms) (a school, from 'to learn'), (a store, from 'to sell'), (a court, from 'to judge'), (a kitchen, from 'to cook'), (a kennel, from 'dog'), (a desert, from 'without water'); (provenance, from 'to come from'); (the appropriate place) having a propensity, tendency (playful), (talkative), (credulous, from 'believe'); (flammable, from 'burn'); (inclination); (unwilling, with ) is a borrowing from Ido. It is often equivalent to the nonce passive conditional participle: 'payable', 'that which would/should be paid'.mandatory (payable), (required reading) the smallest part (a link, from 'chain'); (a spark, from 'fire'); (a snowflake, from 'snow'), (a stitch, from 'sew'), (a splinter, from 'wood'); (an ommatidium, from 'eye'); (a U.S. state, from 'USA'); (a morpheme, from 'word'); (a crumb etc.) a leader, boss (a school principal [see ]); (a mayor, from 'city'); (a centurion, from 'hundred'); (a president of the United States, from 'USA'); (board of directors, with ) diminutive; sometimes affectionate connotations when used with people (a hut, from 'house'); (a booklet); (lukewarm); (to smile, from 'laugh'); (to crack, fracture, from 'break'); (to simmer, from 'boil'); (slightly) a country named after a geographic feature, and now after an ethnicity [unofficial] (Mexico, from 'Mexico City'); (Nigeria, from 'the river Niger'); (England, from 'English person'); (fatherland, from 'father') [cannot be used as a root , because that means 'something'] male [unofficial] (a bull); (a father); (a male student); (a male) an offspring, descendant (a kitten); (a prince, from 'king'); (a sapling, from 'tree'); (an Israelite); (a kit, pup, kid, etc.); (a clan, tribe, with ) to make, to cause (transitivizer/causative) (to kill, from 'die'); (to clean); (to have built); (to cause) to become (intransitivizer/inchoative/middle voice) (to enjoy oneself); (to be born); (to blush, from 'red'); (to join, from ); (to become) an instrument, a tool (a toy, from 'play'); (a knife, from 'cut'); (a remedy, from 'help'); (a solution, from 'solve'); (a tool); (equipment, set of tools, with ) female (a cow); (a mother); (a female student); (a female) worthy of (memorable); (credible, from 'believe'); (dependable, trustworthy, from 'trust'); (something to cry about, from 'weep, cry' and ); (worthy) a holder, sheath (a scabbard, from 'sword'); (a candle-holder); (a tooth socket); (stirrup, from 'foot'); (a cartridge, from 'bullet'); (a socket, etc.) a doctrine, system (as in English) (Communism); (Christianity); (an ism) person professionally or avocationally occupied with an idea or activity (a narrower use than in English) (teacher); (dentist); (a beekeeper); (a communist); (a member of parliament/congress) feminine affectionate form; the root is truncated (Joanie); (mommy); (granny); (aunty); (nanny, from 'nurse'); (dear friend); (the 'girls', from or ) multiple (double); (triply); (more than once) fraction (half [of]); (one hundredth); (a tithe, from 'ten' and ); (a fraction); (to divide into equal parts, with ) in a collective group of specific number (two together; by twos = ); (a trilogy); (how many together?); (together in a group, from ); (drop by drop, from 'drop'; = ); (isolated, individual); (a group, unit, team); (collective) a (loose) container, country (archaic when referring to a political entity), a tree of a certain fruit (archaic) (a purse, from 'money'); (a saltshaker, from 'salt'); (a washbasin, from 'wash'); (a beehive, from 'bee'); (England [ in current usage]); (Kurdistan, the Kurdish lands); (appletree [now ]); (a container) one characterized by the root (a youth); (a saint, from 'holy'); (a beginning reader [student, not book], from "ABC's"); (a mammal, from 'breast'); (a neighbor, from 'near'); (a woman with many children, from 'many' and 'child'); (someone without merit, from 'without' and the suffix ); ~ (a wretch, from the suffix ); (a contemporary, from 'then'); (someone who asks 'why' a lot, from 'why'); (a wisp of a girl); (a fellow) undefined ad hoc suffix (used sparingly: see list) (a collar, from 'neck'); (to crucify, from 'cross'); (a cold, from 'cold'); (to fulfill, from 'full'); (to hug, from 'arm'); (to woo, from 'lovable' [see ]); (clockwise, from 'right'); (appellate court, from 'court(yard)'); (high society, from 'world'); (a community, from 'common'); (approximately, from 'near'); (hexadecimal, from '16'); (a thingamajig)

List of prefixes