at a previous time
"at one time he loved her"
"her erstwhile writing"
"she was a dancer once"
One and only one time.
"He never made that mistake again after he fell once."
In plain English: Once means just one time, happening only a single time.
"Once I finish my homework, I will go outside to play."
Usage: Use "once" to indicate that an action happened exactly one time in the past or could happen only if a specific condition were met. Avoid using it to mean "at any time" or "formerly," which are incorrect interpretations of this frequency adverb.
As soon as; when; after.
"Once the rain stopped, we finally stepped outside to enjoy the cool air."
Once comes from Old English ǣnes, which originally meant "one" and was formed by adding a suffix to the word for one. Its meaning evolved over time in Middle English to indicate a single occurrence.