for one Definition

one
noun

1a. The cardinal number 1;
1b. The quantity that this represents, being a single unit.
2. A unity or unit.
3. Any symbol for this, eg 1, I.
4. The age of one.
5. Something, especially a garment or a person, whose size is denoted by the number 1.
6. The first hour after midnight or midday.

Example: Come at one o'clock
Example: 1pm
7. A score of one point.
8. colloq
A story or joke
Example: heard the one about the singing policeman?
9. colloq
An enthusiast of the specified thing.
Example: She's quite a one for chess
Form: one for something
10. colloq
A drink, especially an alcoholic one.
Example: drop in for a quick one
11. colloq
A daring, remarkable or cheeky person
Example: You are a one!

adj

1. Being a single unit, number or thing.

Thesaurus: whole, entire, total, complete; indivisible, integral, united.
2. Being a particular person or thing, especially as distinct from another or others of the same kind.

Example: lift one leg and then the other
3. Being a particular but unspecified instance or example.

Example: visit him one day soon
4. Being the only such.

Example: the one woman who can beat her
5. Same; identical.

Example: of one mind
Thesaurus: identical, compatible, equal.
6. Undivided; forming a single whole.

Example: They sang with one voice
7. First.

Example: page one
8. colloq
An exceptional example or instance of something.
Example: That was one big fellow
Thesaurus: peculiar, singular, lone, single, solitary, unique, different.
9. Totalling one.
10. Aged one.

pronoun

1. (often referring to a noun already mentioned or implied) an individual person, thing or instance.

Example: buy the blue one
Thesaurus: somebody, someone, person, individual, item, unit, thing, example.
2. Anybody.

Example: One can't do better than that
3. formal or facetious
I; me.
Example: One doesn't like to pry

Idiom: all one

Just the same; of no consequence.
Example: It's all one to me

Idiom: at one with someone or something

In complete agreement with them.

In harmony with them.
Example: at one with nature

Idiom: be one up on someone

colloq
To have an advantage over them.

Idiom: for one

As one person.
Example: I for one don't agree

Idiom: all in one

Together; combined; as one unit or object, etc.

In one go or attempt.

Idiom: just one of those things

An unfortunate event or situation that must be accepted.

Idiom: one and all

Everyone without exception.

Idiom: one and only

Used for emphasis: only.

Idiom: one another

Used as the object of a verb or preposition when an action takes place between two (or more than two) people, etc.
Example: Chris and Pat love one another

Idiom: one by one

One after the other; individually.

Idiom: one or two

colloq
A few.

Etymology: Anglo-Saxon an.

Info:

See Usage Note at each.