Why was "tantrum" trending last week?
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.