Eighteenth is the ordinal form of the number 18. Eighteenth or 18th may also refer to:
A fraction, , equal to one of 18 equal parts 18th of the month, a recurring calendar date 18th birthday, the age of majority in a substantial majority of countries
Geography 18th meridian east, a line of longitude 18th meridian west, a line of longitude 18th parallel north, a circle of latitude 18th parallel south, a circle of latitude 18th Avenue (disambiguation) 18th Street (disambiguation)
Military 18th Army (disambiguation) 18th Battalion (disambiguation) 18th Division (disambiguation) 18th Regiment (disambiguation) 18th Squadron (disambiguation)
Others Eighteenth Amendment (disambiguation) Eighteenth Amendment to the United States Constitution 18th century 18th century BC 18th Congress (disambiguation) 18th Newtownabbey Old Boys, a football club from Northern Ireland
See also 18 (disambiguation)