At once

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.
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.
At the same time, as in We can’t all fit into the boat at once. [ First half of 1200s ]
Also see: at one time, def. 1.
Immediately, as in Mother told the children to come inside at once. [ First half of 1500s ]

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