In "sissy role play" a sissy is a person (typically a man) who adopts hyper-feminine behaviors, and engages in stereotypical "feminine" activities (e.g., housekeeping, putting on make-up), often within the context of BDSM. Particularly within the BDSM community, a "sissy" will typically assume the submissive role to a dominant female and/or male partner(s). Often sissies in a BDSM context also identify with other fetishes and sexual practices.
As a lifestyle desire or fetish activity, a sissy's feminization is an involuntary transformation from masculine to feminine, either physically, behaviorally, or both. The process is known as feminization or sissification. This transformation can either be permanent, or temporary. Additionally, this transformation can be something that the "sissy" goes through only in private (temporarily - fetish) or in public, temporarily or permanently.