Source : Free On-Line Dictionary of Computing

three-tier
     
         A {client-server} architecture in which the
        {user interface}, functional process logic ("business rules")
        and data storage and access are developed and maintained as
        independent {modules}, most often on separate {platforms}.
     
        Apart from the usual advantages of modular software with well
        defined interfaces, the three-tier architecture is intended to
        allow any of the three tiers to be upgraded or replaced
        independently as requirements or technology change.  For
        example, an upgrade of desktop {operating system} from
        {Microsoft Windows} to {Unix} would only affect the {user
        interface} code.
     
        Typically, the user interface runs on a desktop {PC} or
        {workstation} and uses a standard {graphical user interface},
        functional process logic may consist of one or more separate
        modules running on a {workstation} or application {server},
        and an {RDBMS} on a database server or {mainframe} contains
        the data storage logic.  The middle tier may be multi-tiered
        itself (in which case the overall architecture is called an
        "n-tier architecture").
     
        (1998-05-13)