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.
Techopedia explains Thick Client
A thick client is a type of client device in client-server architecture that has most hardware resources on board to perform computation operations, run applications and perform other functions independently. Although a thick client can perform most operations, it still needs to be connected to the primary server to download programs and data, and to update the operating system.
Thick clients are generally implemented in computing environments when the primary server has low network speeds, limited computing and storage capacity to facilitate client machines, and/or there is a need to work offline.