at a time—
- We present items, three at a time, and ask informants to pick the one that is most different from the other two.
2. is used of a particular period of time:
- She grew up in the capital at a time when its artistic life was at its most brilliant.
Note: The expression does not correlate in meaning with the phrase at times—now and then:
- Our work is cut out for us, even though it can be heartbreaking at times.
at one time—
1. = at a time 1:
- Let’s start the dance again, all at one time.
2. at some time in the past:
- At one time most school teachers were men.