Decomposition, n.
Etymology: [Pref. de- (in sense 3 intensive) + composition: cf. F. décomposition. Cf. Decomposition.]
1. The act or process of resolving the constituent parts of a compound body or substance into its elementary parts; separation into constituent part; analysis; the decay or dissolution consequent on the removal or alteration of some of the ingredients of a compound; disintegration; as, the decomposition of wood, rocks, etc.
2. The state of being reduced into original elements.
3. Repeated composition; a combination of compounds. [Obs.] Decomposition of forces. Same as Resolution of forces, under Resolution. -- Decomposition of light, the division of light into the prismatic colors.
Shades of meaning
Fernald lists decomposition only as an antonym, under union.
He lists it without comment; the entries above explain its neighbors.