A fat application is a software application installed in a networked computer that is able to function independently from the central server in a client-server architecture. Rather than circulated over the network, the resources are stored locally in hard drives and other software applications.
Although it still requires a bit of a network connection, fat applications are usually defined by their ability to perform many functions, even without a network or Internet connection.
A fat application is also known as a fat client or thick client.
Techopedia explains Fat Application
For all intents and purposes, a fat client is a sort of stand-alone program that occasionally needs o connect to a server in order to sync data or upload and download instructions. Its alter ego is the thin client, which is dependent on a network connection for the distribution of resources. It is designed to be small so that the heavy job of processing is being done on the server. Since its storing capacity is also limited, many users still prefer the fat client.
Fat applications have the following advantages:
Lower Server Requirements: Since the fat client does most of the application processing by itself, it does not rely mainly on servers for processing.
Working Offline. Fat clients, as mentioned, do not demand a network connection to function.
More Flexibility, Use of Existing Infrastructure: Most software is already designed to contain local resources, especially for personal computers. Because better PCs are now common for end users, they are already built to have the infrastructure to run fat clients.