1. [ [Undefined_variable] ] determiner, one or the other indifferently
2. [ adverb ] 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"

3. [ conjunction ] conjunction, alternative
Synonyms: or

"Either your sister or your friend has been in the garden."

Similar spelling:   eider