Gender play is a practice where an individual is encouraged or forced to take on the role of the opposite gender. Gender play can be a form of punishment or a way for a person to live out fantasies of being the opposite gender. Gender play is most common within the BDSM community, but may be an element of any couple’s sexual relationship, regardless of sexual orientation.
Gender play that involves men take on a female role is called feminization. Sissification is a form of feminization. While not as commonly used, the term masculinization could be used to describe the process of a female taking a male role.