STD /ˌes tiː ˈdiː/ noun [countableC, uncountableU] medical
MI (sexually transmitted disease) a disease that one person passes to another through having sex, such as AIDS or herpes 性传播疾病〔如艾滋病、疱疹〕
From Longman Business Dictionary
STDSTD noun [uncountableU]
subscriber trunk dialling; in Britain, the name for the former telephone system that allowed people to connect their own long-distance calls by using an STD code (=a special number for the area you are calling). Officially, STD codes are now called area codes although many people still use the old name