One (number)

1. A single person or thing.
2. Used to refer to a person or thing just mentioned or easily identified.
3. Used to emphasize a particular person or thing.
4. Used to refer to the speaker or writer.
5. Used to indicate a particular day or time.
6. Used to emphasize the importance or uniqueness of something.

Origin:

Old english ān, of germanic origin; related to dutch een and german ein.

Examples:

1. I have one sister.
2. Can I have the red one?
3. This one is my favorite.
4. One must always tell the truth.
5. I'll see you on Monday, the 14th, at one o'clock.
6. One of a kind, this painting is worth millions.