once
1 at one time, formerly, in the old days, in the past, in times gone by, in times past, long ago, once upon a time, previously
a. directly, forthwith, immediately, instantly, now, right away, straight away, straightway (archaic) this (very) minute, without delay, without hesitation
b. at or in one go (informal) at the same time, simultaneously, together
3 once and for all conclusively, decisively, finally, for all time, for good, for the last time, permanently, positively, with finality
4 once in a while at intervals, at times, every now and then, from time to time, now and again, occasionally, once in a blue moon (informal) on occasion, sometimes
English Collins Dictionary - English synonyms & Thesaurus  

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"Collins Cobuild English Dictionary for Advanced Learners 4th edition published in 2003 © HarperCollins Publishers 1987, 1995, 2001, 2003 and Collins A-Z Thesaurus 1st edition first published in 1995 © HarperCollins Publishers 1995"