To a
transsexual, the
process of changing from living the
life of the
birth gender to that of the
chosen gender.
Usually considered to begin when the person starts making changes with the intent of altering their appearance to more accurately reflect who they want to be. This may be something as simple as a change in hairstyle, or more radical, depending on the person.
When this period is considered to have ended varies from person to person, but is generally the point where there really isn't anything much left to change (at least to the people they see on a regular basis). However, it is, to almost everyone, definitely over by the time of sex reassignment surgery. There are a few people who would say that they consider transition to be a never ending process, but those people are in the minority.