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.