once

[1]

at once , immediately , straight away , right away , directly , without delay , promptly , instantly , post-haste , in a wink , in the twinkling of an eye , in a minute/moment/second/split second , in no time (at all) , before you can turn around , before you can say 'Jack Robinson' , in a trice , (Colloq) in a jiffy , in two shakes of a lamb's tail together , at the same time , simultaneously , at a stroke , in the same instant , in the same breath , (Colloq) at one go , at a go , in one go

conj.

[1]	(if) ever , as soon as , at any time

adv.

[1]	one time , once upon a time , formerly , in one case , (at) one time , on a former occasion , previously , before , in days gone by , in olden days , in the (good) old days , long ago , some time ago , years/ages/aeons ago , in days of yore
[2]	one time , on one occasion , a single time
[3]	once and for all , formerly , at one time , finally , positively , erstwhile , erst , definitely , decidedly , conclusively , for good
[4]	once in a while , occasionally , (every) now and then , now and again , at times , sometimes , periodically , from time to time , at intervals , sporadically