1610s, "the growing together of parts," from French coalition (1540s), from Late Latin coalitus "fellowship," originally past participle of Latin coalescere (see coalesce). First used in a political sense 1715.
An alliance of political groups formed to oppose a common foe or pursue a common goal.
Note: In countries with many political parties, none of which can get a majority of the citizens' votes, the only way an effective government can be formed is by a coalition of parties. Such coalitions are often unstable.