originally, an interrogative pronoun, later, a relative pronoun also; -- used always substantively, and either as singular or plural.  see the note under what, pron., 1.  as interrogative pronouns, who and whom ask the question: what or which person or persons? who and whom, as relative pronouns (in the sense of that), are properly used of persons (corresponding to which, as applied to things), but are sometimes, less properly and now rarely, used of animals, plants, etc.  who and whom, as compound relatives, are also used especially of persons, meaning the person that; the persons that; the one that; whosoever.-one; any; one.