Non con is the shortened version of non-consensual and at it's most basic definition, refers to any sexual act in which the consent of one or both parties is suspect or not explicit.
As a term, it has a rather contentious existence, since in most cases non-consensual is equated with without consent which translates for many people as simply rape. However, unlike the term rape, Non con has no legal recognition in the justice system of the USA (although it may be used elsewhere).