a single person or thing, viewed as taking the place of a group.

(One never knows, does one?)
You and they are also used in a similar way. However, one and you include the speaker in the generalisation One’s is a possessive determiner:
One’s health is much more important than having lots of money.

being or amounting to a single unit or individual or entire thing, item, or object rather than two or more; a single:one woman; one nation; one piece of cake.

being a person, thing, or individual instance or member of a number, kind, group, or category indicated:one member of the party.

Definition:
existing, acting, or considered as a single unit, entity, or individual.

of the same or having a single kind, nature, or condition:We belong to one team; We are of one resolve.

noting some indefinite day or time in the future:You will see him one day.

a certain (often used in naming a person otherwise unknown or undescribed):

means goodbye, spoken as a true gangsta

To close out a meeting, dialogue or some instance of communication with another/others by equating this term with "take care."

A unit of sex, although with no concrete definition. This can be given or slipped.

another word for 'peace'

A great answer to give in math class when a teacher asks you a question to make sure you are paying attention, and you don't know it because you really weren't paying attention.

Great because it is more likely to be the right answer than any other number. Also, even if you're wrong, everybody will think it was just a joke and you will come out on top.

Do not try in other classes or you will look like a dumbass.

1) When a vehicle is nearly out of gasoline, dictated by the gas meter pointing in the general direction of the "E"

Often used as an excuse to not drive somewhere, much more prevalent since gas prices have become greater than $2/gal.