1.  at once(r = adv.all) directly, forthwith, immediately, instantly, like a shot, now, right away, straight off, straightaway - without delay or hesitation; with no time intervening; "he answered immediately"; "found an answer straightaway"; "an official accused of dishonesty should be suspended forthwith"; "Come here now!"

2.  at once(r = adv.all) at a time, at one time - simultaneously; "he took three cookies at a time"

N  instantaneity, instantaneousness, immediacy, suddenness, abruptness, moment, instant, second, minute, twinkling, trice, flash, breath, crack, jiffy, coup, burst, flash of lightning, stroke of time, epoch, time, time of day, time of night, hour, minute, very minute, very time, very hour, present time, right time, true time, exact correct time, instantaneous, momentary, sudden, immediate, instant, abrupt, discontinuous, precipitous, precipitant, precipitate, subitaneous, hasty, quick as thought, quick as lightning, quick as a flash, rapid as electricity, speedy, quick, fast, fleet, swift, lively, blitz, rapid (velocity), instantaneously, in no time, in less than no time, presto, subito, instanter, suddenly, at a stroke, like a shot, in a moment, in the blink of an eye, in the twinkling of an eye, in a trice, in one's tracks, right away, toute a l'heure, at one jump, in the same breath, per saltum, uno saltu, at once, all at once, plump, slap, at one fell swoop, at the same instant, immediately, extempore, on the moment, on the spot, on the spur of the moment, no sooner said than done, just then, slap-dash, touch and go, no sooner said than done.

N  earliness, morning, punctuality, promptitude, haste, suddenness, prematurity, precocity, precipitation, anticipation, a stitch in time, early, prime, forward, prompt, summary, premature, precipitate, precocious, prevenient, anticipatory, rath, sudden, unexpected, near, near at hand, immediate, early, soon, anon, betimes, rath, eft, eftsoons, ere long, before long, shortly, beforehand, prematurely, precipitately, too soon, before its time, before one's time, in anticipation, unexpectedly, suddenly, before one can say 'Jack Robinson', at short notice, extempore, on the spur of the moment, on the spur of the occasion, at once, on the spot, on the instant, at sight, offhand, out of hand, a' vue d'oeil, straight, straightway, straightforth, forthwith, incontinently, summarily, immediately, briefly, shortly, quickly, speedily, apace, before the ink is dry, almost immediately, presently at the first opportunity, in no long time, by and by, in a while, directly, no sooner said than done, immediately, if not sooner, tout vient a temps pour qui sait attendre.

all at once, give the once over, once, once again, once and for all, once in a while, once more