These encyclopedia articles provide a working definition:
These books will help you understand the how to analyze literature, as well as define key literary theories.
(Hint: Read the descriptions of each book to know what literary theories they contain.)
Critical Theory Today: A User-Friendly Guide (3rd ed)
by
Lois Tyson
Introducing Literary Theories: A Guide and Glossary
by
Julian Wolfreys (Editor)
Literary Theory: The Basics
by
Johannes Willem Bertens