See Multi-tier Architecture.

Three-tier architecture is a special type of client/server architecture consisting of three well-defined and separate processes, each running on a different platform: The user interface, which runs on the user's computer (the client). The functional modules that actually process data. This middle tier runs on a server and is often called the application server. 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: The added modularity makes it easier to modify or replace one tier without affecting the other tiers. Separating the application functions from the database functions makes it easier to implement load balancing.

a model of large scale database applications that adds a middle level to the older client/server model in order to make more efficient use of limited back-end database server resources. Large databases require efficient access to many users, often spread out over a wide geographic area or a huge corporate network. The 3-tier design is especially well suited to Internet/intranet applications, because it allows the burden of message processing to be separated from database processing.