Deliberately baiting someone in to reacting to your viewpoint when you don't actually hold the opinion at all.
For instance, coming in to a thread discussing PC games and saying "Why do people bother spending £500+ on a PC when you can get better graphics on a console for £200
![Confused :confused: :confused:](/styles/default/xenforo/vbSmilies/Normal/confused.gif)
". The intent is to make people react to the post, not realising that isn't in the least bit sincere and it's whole purpose was to cause that exact reaction.
Walking up to someone in the street and slapping them round the face is not trolling. Annoying someone overtly is not trolling. It's about subtlety and making people dig their own holes.