- Something that is not real but you wish it was
Fantasy is very important in porn. Porn allows people to see others fuck, get naked. or expose themselves in public for example. All of these are either taboo, forbidden, or give the viewer a secret rush. Watching your partner fuck someone else, showing off in public (flashing, fucking in public, nudity), or seeing a hot girl like the one you work with or whatever naked fulfills a fantasy.
Porn is about both fulfilling the fantasy and giving a feedback loop that reinforces that fantasy and makes it even stronger. It's how porn ends up with niche products, from schoolgirls to spanking, from Fat Girls to domination, and from older women to barely legal virgins. All of those are fantasies for some, and that is what porn is all about!