Interchangeable, a.
Etymology: [Cf. OF. entrechangeable.]
1. Admitting of exchange or mutual substitution. "Interchangeable warrants." Bacon.
2. Following each other in alternate succession; as, the four interchangeable seasons. Holder. -- In`ter*change"a*ble*ness, n. -- In`ter*change"a*bly, adv.
Shades of meaning
Fernald has no entry of its own for interchangeable; he lists it under mutual, synonymous.
Where he writes about it:
Synonymous (Gr. syn, together, and onyma, name) strictly signifies being interchangeable names for the same thing, or being one of two or more interchangeable names for the same thing; to say that two words are synonymous is strictly to say they are alike, equivalent, identical, or the same in meaning; but the use of synonymous in this strict sense is somewhat rare, and rather with reference to statements than to words. — from synonymous
H. W. BEECHER Royal Truths p. 294. [T. & F. '66.] In the strictest sense, synonymous words scarcely exist; rarely, if ever, are any two words in any language equivalent or identical in meaning; where a difference in meaning can not easily be shown, a difference in usage commonly exists, so that the words are not interchangeable. — from synonymous
To consider synonymous words identical is fatal to accuracy; to forget that they are similar, to some extent equivalent, and sometimes interchangeable, is destructive of freedom and variety. — from synonymous