Sexual Reassignment Surgery. Specifically, changing the genitals from female to male or vice versa. Sometimes referred to as GRS for gender reassignment surgery. While chest surgery for FtMs is not technically SRS, sometimes it's lumped in the category too. Only 3% of trans people go through SRS.
I personally think the terminology is misleading because it implies that our sex equals our genitals. See Objection: Sex Cannot Change.
Trans & Gender 101 >