a medical operation to change the appearance of someone's chest that is done as part of the process of gender reassignment (= changing someone's body to match their gender identity). It can involve either removing breast tissue for a trans man, or making the breasts larger for a trans woman.

I always knew that top surgery would be the next stage in my transition. 我一直都很清楚上半身变性手术是我变性过程的下一步。