As a Verb or Adjective:
1. To be any one of two possible alternatives, choices or answers. English = (be) either.
As Modifier:
1. Used to link two alternatives, to give a second alternative, or to introduce a comment which corrects or modifies something you have just said. English = or.
As Question:
1. Used to ask questions when there are two or more possible answers or alternatives. English = which?