Transition usually happens before any sex reassignment surgery, and in some cases even before any hormone replacement therapy. Transition often marks the start of the real life experience, which is usually required for GRS.
Many people also use the term transition to refer to the entire Transgender/transsexual process (from living 24/7 in the original gender role to after surgery). The beginning of the real life experience is then often called 'going full time' (starting to live 24/7 in the opposite gender role).
The process of moving (in presentation and/or lifestyle) from one gender role to another (or potentially out of prescribed gender roles altogether, although this use of the term is unusual).
The term transitioning is often used amongst members of the Trans community to distinguish people who transition full time into a different social gender role from those who do not.