Sissification refers generally to the feminization of a person who identifies as male.
The term was originally used in the context of BDSM to refer to the forced feminization or gender reassignment of a male-identifying submissive. As with any activity, each party has to give their enthusiastic consent and have the right to withdraw or revoke the consent at any time, for any reason.
Through sissification, the submissive becomes more effeminate and exhibits feminine behaviors. The dominant partner takes control of this process. The submissive is "forced" into cross-dressing and gender reassignment. They may get asked to try traditionally feminine activities, such as performing housework or applying makeup. They may transform their bodies by tucking their penis between their legs, removing their body hair, or wearing prosthetic breasts. They may also adopt a feminine voice and name.
As sissification reinforces the dominant and submissive roles of a BDSM couple, the process can be very sexually arousing. Sexual play can be a component of sissification. For example, rather than having penis-in-vagina sex, a dominant may use a strap-on to penetrate their partner anally. This puts the submissive in a more "feminine" sexual role. The dominant may also stop the submissive from touching their penis for sexual gratification.
The term sissification has also evolved to refer to the loss of hyper-masculinity in society. Generally, the term sissy refers to a male-identifying person that does not conform to traditional gender roles. Sissy is thought of as the male equivalent of tomboy although it traditionally has more offensive connotations. Strength and heroism seemed masculine while being a sissy was weak. However, as modern society has shone a light on toxic masculinity, some male-identifying people aim to distance themselves from traditionally masculine traits and become sissies.
Sissification should not be confused with male-to-female transition or feminine presentation in nonbinary people. While these people may present or act in a more feminine way, when previously they presented or acted more masculinely, they lead this change. It is not a result of the influence of a more dominant person or society, as sissification is. People also undergo this type of change to align their gender presentation with their gender identity. It is not for sexual gratification, or for the gratification of others, although the confidence it brings may increase sexual satisfaction.