det,pron

 

one of two one of two people or things when it is not important which

"Would you like red or white wine?" - "Oh, either."

 

both both

People were smoking on either side (= at both sides) of me.

 

adverb

 

used in negative sentences to mean that something else is also true

The menu is boring and it's not cheap either.

 

conjunction

 

used when you are giving a choice of two or more things

Either call me tonight or I'll speak to you tomorrow.