Webster's Revised Unabridged Dictionary · 1913

Once

Once (ons) , noun

(Zoology) The ounce.

Once (wuns) , adverb

[Old English ones, anes, an adverbial form from one, on, an, one. See One-, -Wards.]

1.
For one time; by limitation to the number one; not twice nor any number of times more than one.
Ye shall... go round about the city once. — Josh. vi. 3
Trees that bear mast are fruitful but once in two years. — Bacon
2.
At some one period of time; -- used indefinitely.
My soul had once some foolish fondness for thee. — Addison
That court which we shall once govern. — Bp. Hall
3.
At any one time; -- often nearly equivalent to ever, if ever, or whenever; as, once kindled, it may not be quenched.
Wilt thou not be made clean? When shall it once be? — Jer. xiii. 27
To be once in doubt Is once to be resolved. — Shakespeare

Once is used as a noun when preceded by this or that; as, this once, that once. It is also sometimes used elliptically, like an adjective, for once-existing. “The once province of Britain.” J. N. Pomeroy.

Collocations (2)
At once , (a) At the same point of time; immediately; without delay. Stand not upon the order of your going, but go at once. — Shakespeare I... withdrew at once and altogether. — Jeffrey (b) At one and the same time; simultaneously; in one body; as, they all moved at once.
Once and again , once and once more; repeatedly. A dove sent forth once and again, to spy. — Milton