1.  Simultaneously, at the same time, as in All the boys jumped into the pool at one time. For synonyms, see at once, def. 1; at the same time, def. 1.
2.  Formerly, in the past, as in At one time very few houses in town were on the market.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.