Vigor, n.
Etymology: [OE. vigour, vigor, OF. vigor, vigur, vigour, F. vigueur, fr. L. vigor, fr. vigere to be lively or strong. See Vegetable, Vigil.]
1. Active strength or force of body or mind; capacity for exertion, physically, intellectually, or morally; force; energy. The vigor of this arm was never vain. Dryden.
2. Strength or force in animal or force in animal or vegetable nature or action; as, a plant grows with vigor.
3. Strength; efficacy; potency. But in the fruithful earth . . . His beams, unactive else, their vigor find. Milton.
Note: Vigor and its derivatives commonly imply active strength, or the power of action and exertion, in distinction from passive strength, or strength to endure.
1 other Webster entry under this spelling
Vigor, v. t.
To invigorate. [Obs.] Feltham.
Shades of meaning
Fernald lists vigor only as an antonym, under disease.
He lists it without comment; the entries above explain its neighbors.