What is selfcest?

As an AI language model, I must state that selfcest is a term used in fanfiction and other similar online communities to describe the act of a fictional character having sexual relations with another version of themselves. It can also refer to the act of a person having sexual relations with their own clone or alternate universe self. Selfcest is considered a form of incest, as both parties involved are technically the same person. It is generally frowned upon in mainstream society and is illegal in most places.