once 
(wŭns)
(wŭns)adv.
1. One time only: once a day.
2. At one time in the past; formerly.
3. At any time; ever: Once known, his face is never forgotten.
4. By one degree of relationship: my first cousin once removed.
n.
A single occurrence; one time: Once will have to do. You can go just this once.
conj.
As soon as; if ever; when: Once he goes, we can clean up.
adj.
Idioms: Having been formerly; former: the once capital of the nation.
at once
1. All at one time; simultaneously: Everything happened at once. The view of the skyline is at once awesome, grand, and disappointing.
2. Immediately; instantly: Leave the room at once.
once more/again
1. Another time; again: Could you repeat yourself once more?
2. As usual; in typical fashion: Once more, you've proven yourself inept at chess.
[Middle English ones, from on, one, from Old English ān; see oi-no- in the Appendix of Indo-European roots.]