Longman Dictionary of Contemporary English homepage

Language: Old English
Origin: an

one

3 determiner
     
one3 S1 W1
1 used to emphasize a particular person or thing:
One person I find very difficult is Bob.
If there's one thing I can't stand, it's people who bite their nails.
2

one day/morning/year etc

a) on a particular day, morning etc in the past:
One morning I was sitting at my desk when a policeman knocked at my door.
b) used to talk about a day, morning etc in the future which is not yet exactly known or decided:
We should go out for a drink one evening.
One day she hopes to move to the South Coast.
3 used to talk about a particular person or thing in comparison with other similar people or things:
Why does my card work in one cash machine and not in another?
4

It's one thing to ... it's (quite) another to

used to say that the second thing mentioned is very different from the first, and is often much more difficult to do:
It's one thing to say we have a goal; it's another to actually act on it.
5

for one thing

used to introduce a reason for what you have just said:
He couldn't bring himself to say what he thought. For one thing, she seldom stopped to listen. For another, he doubted that he could make himself clear.
6

be one crazy woman/be one interesting job etc

spoken especially American English to be a very crazy woman, be a very interesting job etc:
You're one lucky guy.
7 formal used before the name of someone you do not know or have not heard of before [= a certain]:
He was accused of stealing a horse from one Peter Wright.

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