Definition of collusion
Collusion is when 2 or more players at the same table conspire with each other to exploit 1 or more other players at the table. Collusion is illegal in all poker rooms, whether online or live. Collusion can take many forms, but the goal is always to act as a group of 2 or more to gain an unfair advantage.
Some examples of collusion
Let's think about a 6 seat online cash game table. You and 4 of your friends take a seat at the table waiting for a 6th player to bust. The goal is to systematically raise as soon as the 6th player is in the hand. So, most of the time, he will have to fold. If that player raises say a pair of 9s, but sees that he is 3bet, and 4bet, he will think he is beaten and fold his hand. The 5 players will then divide the profits between them. Another example of collusion is in live play. You and your friends make a list of non-verbal codes to use with your hands to show your group your two cards in hand. It is forbidden to reveal the nature of your cards during a round. But with this system, you can give your friends valuable information by letting them know your cards without anyone knowing. The examples of collusion are unlimited.