But one question what is the true meaning of colluding?
Collusion is when you work together with someone to disadvantage someone else. In the school yard, it'd be called "ganging up on someone".
The big issue in poker is the difference between
implicit collusion an
explicit collusion.
In the case of implicit collusion, a couple of players will work together to the disadvantage of another without saying (verbally, or in the chat box) what they are doing. The most common example is two players checking a hand down when a third player is all-in. It's collusion, but it hasn't been stated and since nothing can be proved, nothing can be done about it. Most players accept this form of collusion, BTW, because by its nature it only happens in certain very clearly defined circumstances (like that mentioned above) and most will actually expect it to happen.
Explicit collusion, on the other hand, happens when a player or players actually
state their intention to collude. They actually say / type "Lets check it down" in the above all-in situation, or say stupid things like "Lets take turns stealing the blinds of the guy that's sitting out". In these cases, the players are clearly colluding and it can be proved. Oops.