A special type of client/server architecture consisting of three well-defined and separate processes, each running on a different platform:
1. The user interface, which runs on the user's computer (the client).
2. The functional modules that actually process data. This middle tier runs on a server and is often called the application server.
3. A database management system (DBMS) that stores the data required by the middle tier. This tier runs on a second server called the database server.
The three-tier design has many advantages over traditional two-tier or single-tier designs, the chief ones being: