What's the difference between bimestrial and once?

Bimestrial

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Definition:

• (a.) Continuing two months.

Example Sentences:

Once

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Definition:

• (adv.) By limitation to the number one; for one time; not twice nor any number of times more than one.
• (adv.) At some one period of time; -- used indefinitely.
• (adv.) At any one time; -- often nearly equivalent to ever, if ever, or whenever; as, once kindled, it may not be quenched.

Example Sentences: