As a verb, retard (pronounced ruh-TARD) means (1) to cause to move slowly, or (2) to delay or impede. It’s also a musical term meaning a slackening of tempo. Retard as a slang term for any person or group is considered offensive.
The adjective retarded is less offensive when used in reference to medically recognized conditions. But because the term is often abused and has actually been dropped by many medical publications and governing bodies, alternatives such as mentally challenged, mentally handicapped, and developmentally delayed are better.
In general, reputable publications use retarded only in a few narrow contexts—when discussing the offensiveness of the word, in quoted speech, or as the past tense of the verb retard. But, again, the term still has some legitimacy in the medical field, so it does appear occasionally in inoffensive uses.
The doctor prescribed various antihistamines and, to retard the growth of mold in her home, told her to use large fans. [Washington Post]
[A] slavish adherence to emerging market solidarity is likely to retard, rather than advance, progress. [Financial Times]
Some critics blame this for retarding investment infrastructure. [Sydney Morning Herald]