Collusion is a phrase that describes a situation where two or more owners in the same league work together to get an advantage over other participants.
There are different ways in which members of a fantasy football league can conspire together to gain an advantage. It is important to note that each of the listed ways are frowned upon and are considered cheating.
With enough people colluding, fantasy football league rules that are proposed and even accepted can be unfair for the rest of the league.
The most common form of collusion is through trades. Owner A sends a consensus top five player in the league to owner B in return for a player who doesn't even start for a team. The trade is clearly unbalanced and is done so with the intent to work together to better one team.
Collusion can happen for a variety of reasons. It could be two friends that are in a league together. There could be an agreement between two owners that the favored would be repaid in the future. A lot of the time, one owner is already eliminated from making the playoffs and another who is right on the cusp of the playoffs convinces them to help.
However, the one constant is that collusion is always done with the intent to unfairly give someone an advantage.
As with every rule in fantasy football, it varies from league to league. In leagues that are for money the punishment may be more severe than a free one. League managers may decide to rescind a championship or even kick those who committed the offense out of the league.