once in a while Definition

once
adverb

1a. A single time;

Example: I'll say this only once
Thesaurus: one time, this time, never again, a single time; Antonym: repeatedly, frequently.
1b. On one occasion.

Example: They came once
2. Multiplied by one.
3. At some time in the past; formerly.

Example: lived in London once
Thesaurus: formerly, previously, earlier, in the old days, in the past, heretofore, long ago, already, before.
4. Ever; at any time.

Example: if once you are late
5. By one degree of relationship.

Example: hasn't once visited
Example: a cousin once removed

conj

1. As soon as; when once or if once.

Example: Once you have finished you can go out

noun

1. One time or occasion.

Example: just this once

Idiom: all at once

Suddenly.

All at the same time; simultaneously.

Idiom: at once

Immediately; without any delay.
Thesaurus: directly, now, right away, straightaway, this minute, without delay, instantly, unhesitatingly.

All at the same time; simultaneously.

Idiom: for once

On this one occasion if on no other; as an exception.

Idiom: once again (once more)

One more time, as before.

Idiom: once and for all (once for all)

For the last time; now and never again.

Idiom: once in a way (once in a while)

Occasionally; rarely.
Thesaurus: sometimes, occasionally, from time to time.

Idiom: once or twice

A few times.

Idiom: once upon a time

The usual way to begin fairy tales: at a certain time in the past.

Etymology: Anglo-Saxon anes, originally meaning ‘of one', from an one.

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while
conj

1. At the same time as.

Example: She held the bowl while I stirred
2. For as long as; for the whole time that.

Example: guards us while we sleep
3. During the time that.

Example: happened while we were abroad
4. Whereas.

Example: He likes camping, while she prefers sailing
5. Although.

Example: While I see your point, I still cannot agree

adverb

1. At or during which.

Example: all the months while I was ill

noun

1. A space or lapse of time.

Example: after a while

verb

whiled, whiling
1. To pass (time or hours, etc) in a leisurely or undemanding way.

Form: while away something (often)

Idiom: all the while

During all the time that.

Idiom: every once in a while (once in a while)

Now and then; sometimes but not often.

Idiom: for a while

For a long time.
Example: I haven't been to the cinema for a while

Idiom: in a while (in a little while)

In a short time.
Example: I'm going out in a while anyway

Idiom: in between whiles

During the intervals.

Idiom: make it worth someone's while

colloq
To reward them well for their trouble.

Idiom: worth while (worth one's while)

Worth one's time and trouble.

Etymology: Anglo-Saxon hwil.