Federation, n.
Etymology: [Cf. F. fédération.]
1. The act of uniting in a league; confederation.
2. A league; a confederacy; a federal or confederated government. Burke.
Shades of meaning
Fernald has no entry of its own for federation; he lists it under alliance, association.
Where he writes about it:
Federation is mainly a poetic and rhetorical word expressing something of the same thought, as in Tennyson's "federation of the world," Locksley Hall, l. 128. — from alliance
The United States is not a confederacy nor an alliance; the nation might be called a federation, but prefers to be styled a federal union. — from alliance
We speak of an alliance of nations, a club of pleasure-seekers, a community of Shakers, a company of soldiers or of friends, a confederacy, confederation, federation, or union of separate states under one general government, a partnership or company of business men, a conjunction of planets. — from association