S.

Salmacian - One who wishes to have a mixed genital set
Seahorse Dad - Refers to a transmasculine person who chooses to undergo pregnancy.
Sex Assignment - The process by which one's biological sex is determined at birth.
Spironolactone - A testosterone blocker often taken by trans-feminine individuals as a part of HRT.
Stealth - When a transgender person lives and generally passes as their true gender without disclosing that they are trans.
S.T.P. - Stand To Pee (device). A device used to allow one to pee standing up.
Surgeons - Those responsible for "the surgery." Which surgery? It depends on who you ask.