As a Noun: 1. An idea or set of ideas that is intended to explain something. English = theory. 2. A set of ideas that relate to a particular subject. English = theory. As a Verb or Adjective: 1. To be based on or concerning the ideas and abstract principles of a subject, rather than the practical aspects of it. English = (be) theoretical. 2. To develop ideas about something in order to explain it. English = theorise.