|part of speech:
||any one of a group or kind.
A dog has four legs.He needs a doctor!
||one, as a quantity of things.
I bought a bottle of orange juice and two apples.
||one; used to introduce one particular thing or person of a group or kind, but one which is not known to the listener or is not important to identify.
A friend of mine drove me home from work today.We have a dog.