Sulfadimidine or sulfamethazine is a sulfonamide antibacterial.
There are non-standardized abbreviations for it as "sulfadimidine" (abbreviated SDI and more commonly but less reliably SDD) and as "sulfamethazine" (abbreviated SMT and more commonly but less reliably SMZ). Other names include sulfadimerazine, sulfadimezine, and sulphadimethylpyrimidine.
References
Further reading ChemDB. , ChemDB, National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH)