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.