the number 1; used before a noun to say there is a single person or thing
I have one sister.
Please give me one cookie.
the symbol or idea of the number 1; a single person or thing
The number one comes before two.
He rolled a one on the die.
used to refer to a person or thing that has already been mentioned or is known
I like the blue shirt, but I'll buy the red one.
Which apple do you want? This one or that one?
used to talk about people in general, including the speaker and listener
One should always tell the truth.
In winter, one can see snow on the mountains.
