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at one time in the past; formerly:
I was a farmer once; a once powerful nation.
a single time:
We ate there just once. We go to a movie once a week.
even a single time; at any time; ever:
If the facts once become known, it will be just too bad.
by a single step, degree, or grade:
a cousin once removed.
former; having at one time been:
the once and future king.
if or when at any time; if ever.
whenever; as soon as:
Once you’re finished, you can leave.
a single occasion; one time only:
Once is enough.
all at once,

simultaneously:
The children were running, screaming, and throwing things all at once.
suddenly:
All at once the rain came down.

at once,

at the same time; simultaneously:
Don’t all speak at once.
immediately; promptly:
Tell him to come at once!

once and again, repeatedly:
He has been told once and again not to slam the door.
once and for all, decisively; finally:
Let’s settle this problem once and for all.
Also, once for all.
once in a while, at intervals; occasionally:
She stops in to see us once in a while.
once or twice, a very few times; infrequently:
I’ve seen her in the elevator once or twice.
once upon a time, at some unspecified past time, especially a long time ago:
Once upon a time, in a faraway land, there lived a prince and princess.
Contemporary Examples

It only shows, once again, that Jalics has indeed decided to forgive Bergoglio for whatever he did and that he wants to move on.
The Pope’s Dirty Past Christopher Dickey March 15, 2013

FitzPatrick was once one of the heroes of Celtic Tiger Ireland.
The Men Who Killed the Economy Niall Stanage November 18, 2010

The former Virginia governor was once the golden boy of the GOP.
2014 Was a Delectably Good Year for Sleaze Patricia Murphy December 29, 2014

But, as it stands right now, this season of Top Chef has done nothing but sour me on a show I once loved.
Why I’m Tired of ‘Top Chef’ Jace Lacob January 31, 2012

Let’s hope that that the increased presence of women in the new Knesset will help free us from that chauvinism once and for all.
The Latent Chauvinism Of Lapid’s “Zoabiz” Apology Rawia Aburabia February 13, 2013

Historical Examples

For once, he was revealing that fundamental egotism which is the characteristic of all his kind.
Within the Law Marvin Dana

For his sake, I am glad once more to be in my own happy home.
Philothea Lydia Maria Child

She had put it conveniently in her pocket, so that she could place her hand on it at once.
Rico and Wiseli Johanna Spyri

Now, all at once, he saw this to be a world in which dreams come more than true.
The Spenders Harry Leon Wilson

And when I find that which people tell me cannot be done, that I do–at once.
The Leopard Woman Stewart Edward White

adverb
one time; on one occasion or in one case
at some past time; formerly: I could speak French once
by one step or degree (of relationship): a cousin once removed
(in conditional clauses, negatives, etc) ever; at all: if you once forget it
multiplied by one
once and away

conclusively
occasionally

once and for all, conclusively; for the last time
once in a while, occasionally; now and then
once or twice, once and again, a few times
once upon a time, used to begin fairy tales and children’s stories
conjunction
(subordinating) as soon as; if ever or whenever: once you begin, you’ll enjoy it
noun
one occasion or case: you may do it, this once
all at once

suddenly or without warning
simultaneously

at once

immediately
simultaneously

for once, this time, if (or but) at no other time
adv.

c.1200, anes, from ane “one” (see one ) + adverbial genitive. Replaced Old English æne. Spelling changed as pronunciation shifted from two syllables to one after c.1300. Pronunciation change to “wuns” parallels that of one. As an emphatic, meaning “once and for all,” it is attested from c.1300, but this now is regarded as a Pennsylvania German dialect formation. Meaning “in a past time” (but not necessarily just one time) is from mid-13c.

Once upon a time as the beginning of a story is recorded from 1590s. At once originally (early 13c.) meant “simultaneously,” later “in one company” (c.1300), and preserved the sense of “one” in the word; the phrase typically appeared as one word, atones; the modern meaning “immediately” is attested from 1530s.

once and for all
once bitten, twice shy
once in a blue moon
once in a lifetime
once in a while
once over lightly
once upon a time

also see:

all at once
at once
every now and then (once in a while)
give someone the once-over