Collusion
Collusion is two or more players acting with a secret, common strategy. Some common forms of collusion are: soft play, that is, failing to bet or raise in a situation that would normally merit it, to avoid costing one's partner or friend money; whipsawing, where partners raise and re-raise each other to trap players in between; dumping, where a cheater will deliberately lose to a partner; and signalling, or trading information between partners via signals of some sort, like arranging their chips in a certain manner.
In a poker tournament, when one player is all in and two other players are active in the pot, it is common for the two players with chips left to "check it down", or check on each round of betting through the end of the hand. Unless they explicitly communicate an agreement about checking it down, this is not collusion.
Read more about this topic: Cheating In Poker