Short for "nonconsensual" in fanfiction, regular fiction, or just in general referring to various kinds of relationship scenarios where one or both of the participants is forced into sex. Generally associated with rape fantasy and highly controversial.
On the whole, relationships in fanfiction are between two consenting individuals, but there is the occasional noncon.
An individual who consciously goes against what's popular; short for "nonconformist"
"She won't go to the movie theater anymore - she's too much of a noncon."