(after a negative) As well.
I don't like him, and I don't like her either.
One or other of two people or things.
He made me two offers, but I did not accept either.
Both, each of two or more.
Introduces the first of two (or occasionally more) options or possibilities, the second (or last) of which is introduced by “or”.
Either you eat your dinner or you go to your room.