What is the difference between "onetime" and "one time"?

I was reading a book that had a sentence containing this:

...onetime commissioner of New York...

Solution 1:

• onetime, one-time:

  1. former, previous, erstwhile, quondam. So "a onetime commissioner of New York" was a commissioner at some point in the past, but no longer holds that office. I usually see this meaning written without a hyphen.
  2. occurring once, e.g. "hopefully, that was a one-time mistake". IME this meaning is more likely to have a hyphen.

• one time:

  1. once, on one occasion. "Yes, I went there one time."
  2. once upon a time. "One time, in band camp...".
  3. Can also occur in phrases such as at one time (formerly, at a previous time; or sometimes, simultaneously).

Solution 2:

• According to the NOAD, and also to the OALD, it's spelt as "one-time". It's an adjective (so it goes before nouns, usually) and the first meaning is "former".

The other meaning is "of or relating to a single occasion: a one-time charge."

So it can be both. The sentence you provide has not enough context to decide which one of those two is the right meaning for your case.

• About "one time", I don't think it's used, since the correct way to say that is "once", for example:

I've been to that Stadium only once.

Same goes for "two times" which is "twice". These are the only exceptions, after them you say: Three/four/five/etc... times.

(see comments too)

Solution 3:

Strangely, my American Heritage dictionary says that onetime (not hyphenated) means former, and one-time (hyphenated) means only once. I'm not sure I completely believe this distinction, at least in terms of how it is actually used. I suspect many people are completely unaware of this distinction (as I was until I looked it up this morning).

Certainly, when it is used as an adjective (either meaning), it should be spelled onetime or one-time. Otherwise it should be two words.

One time only! A special concert for ...

and

One-time pads are the only truly secure cryptosystems.

John, a onetime professional basketball player ...