Someone who deliberately aggravates or annoys someone by refusing all logic towards any valid point's given in an argument, usually because of stubbornness or general need to be right.
Or un-deliberately by having a general lack of knowledge on the subject while remaining to have a narrow minded discussion and refusing to consider any other opinions, or facts while reassuring themselves with what little knowledge they have.
All of which usually involve from the 'Troll' or 'Troller' after it/they have been proved wrong on one or more occasion.
Rush: You just can't have a thousand folds in a katana
Troll: Nah man, my mate has a katana, he said it had a thousand folds
Rush: but yo...
Troll: nah it just has a thousand fucking folds man
Rush to customer: that guy was such a RL (Real life) Troll