All this data is potentially out of date, and should be taken with a truckload of salt
A coalition is a grouping of alliances and corporations allied together for a common purpose. Coalitions are common in 0.0 space where lone alliances and corporations are highly susceptible to attack from larger entities, so the "safety in numbers" methodology prevails where mainly large alliances form a coalition with smaller entities that all fight towards the same common goal.
Coalitions That Exist or Have Existed
Below is a list of the major coalitions currently in EVE (that have pages on EVElopedia).