Anti-: Prefix generally meaning "against, opposite or opposing, and contrary." In medicine, anti- often connotes "counteracting or effective against" as in antibacterial, anti-infective, and antiviral. Sometimes medical terms containing anti- take on new meanings as has occurred with antibiotic and antibody. As a prefix, anti- may be shortened to ant- as in antacid. "Anti" is the Greek word for "against."
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