A Coalition is an alliance between entities, during which they cooperate in joint action, each in their own self-interest. This alliance may be temporary, or a matter of convenience. A coalition thus differs from a more formal covenant. See Axelrod's book, The Evolution of Cooperation.
In politics, see also Coalition government.
In mathematics, coalitions can be studied as games. The Nash equilibrium defines conditions where rational players can benefit other players in the coalition.
In the computer field, and in the study of cognition, the entities can be called agents or daemons. By definition, agents can form coalitions.