Once

·adv At some one period of time;

— used indefinitely.

II. Once ·adv By limitation to the number one; for one time; not twice nor any number of times more than one.

III. Once ·adv At any one time;

— often nearly equivalent to ever, if ever, or whenever; as, once kindled, it may not be quenched.

once in a while
Occasionally; sometimes. Scarcely a day passes in which from two to half a dozen of our paragraphs ...

Dictionary of American Words And Phrases by John Russell Bartlett.
every once in a while
A singular though very common expression, signifying the same as every now and then. ...

Dictionary of American Words And Phrases by John Russell Bartlett.