Definition:
A thick client is a computing workstation that includes most or all of the components essential for operating and executing software applications independently.
A thick client is one of the components in client-server computing architecture that is connected to the server through a network connection and doesn’t consume any of the server’s computing resources to execute applications. A thick client may also be known as a heavy, fat or rich client.
Further Reading:
Book: The Unified modeling Language user guide by Grady Booch, James Rumbaugh and Ivar Jacobson