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

esp. a long time ago

Once upon a time, in a faraway land, there lived a prince and princess.

adjective, adverb

never

adjective, adverb

already , a single time , at one time , away back , back , back when , before , but once , bygone , earlier , erstwhile , formerly , heretofore , in the old days , in the olden days , in times gone by , in times past , late , long ago , old , once only , once upon a time , one , one time before , one time previously , only one time , on one occasion , previously , quondam , sometime , this time , time was , whilom

adverb

already , before , erstwhile , formerly , previously

adjective

erstwhile , former , old , onetime , past , previous , quondam , sometime , whilom