after a negative statement used as an intensive meaning something like `likewise' or `also'
"he isn't stupid, but he isn't exactly a genius either"
"I don't know either"
"if you don't order dessert I won't either"
As well.
"The project has either increased our revenue as well."
In plain English: Either means you can choose one thing or the other, but not both at the same time.
"I will call you either today or tomorrow."
Usage: Use either to mean "also" or "as well" when adding information about someone else doing the same thing you just mentioned, often following a verb like "do." For example, say "I can't swim, and neither can my brother" instead of using either in this specific affirmative-negative structure.
Any one (of two).
"You can choose either of these restaurants for our dinner tonight."
One or the other of two people or things.
"Sarah can either bake a cake or make cookies for the party tonight."
Introduces the first of two (or occasionally more) options or possibilities, the second (or last) of which is introduced by “or”.
"I can either stay home tonight or go out with my friends."
Either comes from the Old English word for "each one of two," which itself traces back to Proto-West Germanic roots meaning "all" and "which." The term entered Middle English with this same sense of referring to one or the other of a pair.