A Thick Client is a “traditional” application that implements client functionality in a client/server architecture. Unlike a Thin Client that are automatically downloaded from a web page, Thick Client applications must be installed on each individual computer and are typically larger than their Thin Client counterparts. The advantage of Thick Clients over Thin Clients is that they typically have more functionality and are tightly bound to the local system on which they execute. Consequently, they are typically faster too, but this is a practical observation, rather than a technical constraint.
See also: Client, Thin Client