adv(1)(仅仅)一次(one time only)：～a day/a week/a year 一天/周/年一次;～bitten，twice shy (prov) 一朝被蛇咬，十年怕井绳;He'd met her only～but he remembered her.他只见过她一次，却记住了她。Some trees only bear fruit～every twenty years.有些树20年才结一次果。We've been to Europe～.我们去过一次欧洲。〖反〗never，numerously，regularly;

(2)一度，曾经;过去，从前(at a time in the past;formerly)：I～wanted to be a doctor.我曾经想当医生。She was～my best friend.她曾是我最好的朋友。O～dinosaurs walked on these plains.从前恐龙曾在这片荒原上行走。This detective story was～very popular but nobody reads it today.这部侦探小说从前很受欢迎，可是现在没有人看了。I was a little kid～too.我也曾是个小孩子。〖同〗formerly，previously;〖反〗hereafter;

(3)根本，即使一次(ever;at all，even for one time)(同于neg或interr)：I have not seen him here～.我在这里一次也没见到过他。Don't～leave today's work for tomorrow.今日事，今日毕。

all at once 突然：All at～he leaped to his feet.他突然跳了起来。

at once 1)立刻(即)，马上：Come here at～! 马上到这儿来! 2)同时：Everything happened at～.所有的事都一起发生了。

(just) for once;just this once 仅此一次，下不为例：He doesn't usually allow people to leave work early but he'll let you do it just for～.他通常不准许员工早退，但这次他破例让你早走。

get/give sb/sth the once-over 粗略检查或察看：I haven't got much time but I'll give your report the～-over.我时间不多，但我会大致看一看你的报告。

once again;once more 再一次：If you're late～again，you'll lose your job.如果你再迟到的话，就会失去你的工作。

once and for all 一劳永逸地，永远地，彻底地：We have decided to settle the matter～and for all.我们决定一劳永逸地解决这个问题。

once in a blue moon 极少，从不(infml)： Coin collecting is interesting，but you find a valuable coin only～in a blue moon.集硬币很有趣，但是发现有价值的硬币却是难上难。

(every) once in a while 偶尔，间或：They go out together～in a while but not very often.他们偶尔一起出去，但不是很经常。

once or twice 一两次：I don't really know him — I've only met him～or twice.我不大认识他，我只见过他一两次。

once too often 多次幸免(成功)这次遭殃(受挫)：He exceeded the speed limit～too often and was fined.他再次超速开车，这次被罚了款。

once upon a time 从前……;古时候……(用于童话故事开头)：O～upon a time there was a king whose name was Miles... 从前有一个国王名叫迈尔斯……

conj 一旦;一……就……(as soon as;when)：O～you began you could not stop.你一旦开始就不能停止。O～you've learned the method it will be easy.你一旦学会了方法，就会很容易。

n 这一次;一次(the one time;on one occassion)(infml)[U，the/this～]：O～is enough.一次足矣。You must be patient;just this～.你一定要耐心，就这一次。I met her just the～.我见到她仅此一次。