Coalition, n.
Etymology: [LL. coalitio: cf. F. coalition. See Coalesce.]
1. The act of coalescing; union into a body or mass, as of separate bodies or parts; as, a coalition of atoms. Bentley.
2. A combination, for temporary purposes, of persons, parties, or states, having different interests. A coalition of the puritan and the blackleg. J. Randolph. The coalition between the religious and worldly enemies of popery. Macaulay.
Syn. -- Alliance; confederation; confederacy; league; combination; conjunction; conspiracy; union.
Shades of meaning
Fernald has no entry of its own for coalition; he lists it under alliance, union.
Where he writes about it:
Coalition is oftenest used of political parties; fusion is now the more common word in this sense. — from alliance