S.

Salmacian - One who wishes to have a mixed genital set
Scrotoplasty - A type of surgery to create or repair the scrotum.
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.
Sexuality - The quality of being sexual, especially sexual orientation and behavior.
Slurs - Mean words.
Spironolactone - A testosterone blocker often taken by trans-feminine individuals as a part of HRT.
S.R.S. - Sex Reassignment Surgery
Stealth - When a transgender person lives and generally passes as their true gender without disclosing that they are trans. This can be done either for personal or safety reasons (or a combination of both).
Stonewall - A riot of much historical importance.
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.
Synthetic Hormones - Hormones with a chemical structure that differs from the natural hormones produced by the body. These are produced in a lab.