adv. 25B6; adverb
I only met him once : ON ONE OCCASION, one time, one single time.
he did not once help : EVER, at any time, on any occasion, at all.
they were friends once : FORMERLY, previously, in the past, at one time, at one point, once upon a time, time was when, in days/times gone by, in times past, in the (good) old days, long ago; archaic sometime, erstwhile, whilom; poetic/literary in days/times of yore.
often, now.
25B6; conjunction he'll be all right once she's gone : AS SOON AS, when, after.
25A0; at once
you must leave at once : IMMEDIATELY, right away, right now, this moment/instant/second/minute, now, straight away, instantly, directly, forthwith, promptly, without delay/hesitation, without further ado; quickly, as fast as possible, as soon as possible, a.s.a.p., speedily; informal like a shot, in/like a flash, before you can say Jack Robinson.
all the guests arrived at once : AT THE SAME TIME, at one and the same time, (all) together, simultaneously; as a group, in unison, in concert, in chorus.
25A0; once and for all CONCLUSIVELY, decisively, finally, positively, definitely, definitively, irrevocably; for good, for always, forever, permanently.
25A0; once in a while OCCASIONALLY, from time to time, (every) now and then/again, every so often, on occasion, at times, sometimes, off and on, at intervals, periodically, sporadically, intermittently.