Conditional, a.
Etymology: [L. conditionalis.]
1. Containing, implying, or depending on, a condition or conditions; not absolute; made or granted on certain terms; as, a conditional promise. Every covenant of God with man . . . may justly be made (as in fact it is made) with this conditional punishment annexed and declared. Bp. Warburton.
2. (Gram. & Logic)
Expressing a condition or supposition; as, a conditional word, mode, or tense. A conditional proposition is one which asserts the dependence of one categorical proposition on another. Whately. The words hypothetical and conditional may be . . . used synonymously. J. S. Mill.
1 other Webster entry under this spelling
Conditional, n.
1. A limitation. [Obs.] Bacon.
2. A conditional word, mode, or proposition. Disjunctives may be turned into conditionals. L. H. Atwater.
Shades of meaning
Fernald lists conditional only as an antonym, under absolute.
He lists it without comment; the entries above explain its neighbors.