The protagonist or one of the heroes in this story starts as a male but has his lifestyle modified into that of a trap/crossdresser by his own choice or by sexual training, either consensual or not. The key point of this tag is not about appearance but it is about taking on tasks, behaviours and roles that are overtly feminine, and adopt female mannerisms and postures.
All of that usually ends up in cross-dressing, especially in the clothes related to a traditional submissive female role, such as schoolgirl, secretary or a maid. Surgical intervention and/or sex change might eventually occur, but is not a requirement.
Where feminization is forced, it also usually leads to erotic humiliation, corporal punishment and cuckolding.
What NOT to consider feminization:
- VN where a character wakes up in a girl's body or he magically gender bends
- VN where a character disguises as a cross-dresser for various purposes, like infiltrating an all-girl school
- VN where the character is transgender and assumes feminine role as part of their transitioning
An example of a VN about feminization is (NSFW): v19290.
If in doubt if the tag applies, note that Feminization is a fetish and thus is employed for the gratification of either feminizee or the feminizer.