Once  —adv. 1 on one occasion only. 2 at some point or period in the past. 3 ever or at all (if you once forget it). 4 multiplied by one. —conj. As soon as. —n. One time or occasion (just the once).  all at once 1 suddenly. 2 all together. At once 1 immediately. 2 simultaneously. For once on this (or that) occasion, even if at no other. Once again (or more) another time. Once and for all (or once for all) in a final manner, esp. After much hesitation. Once (or every once) in a while from time to time. Once or twice a few times. Once upon a time at some unspecified time in the past. [originally genitive of *one]