What does the abbreviation STD mean?

STD stands for Sexually Transmitted Disease.

S.T.D. with periods stands for Doctorate of Sacred Theology. most famously seen on the cover of the Strong's concordance.

In the context of telephone codes, STD stands for "Subscriber Trunk Dialling," which is to say intercity calls dialed directly by the customer without operator assistance. "STD Code" is another term for telephone area code.

On a paycheck, STD can stand for short term disability.

STDev can also stand for "standard" or "standard deviation"
STD Means Standard Trunk Dialing
Sexually Transmitted Disease.
If by that you mean "what does STD stand for?", it means Sexually Transmitted Disease.