from
The Free On-line Dictionary of Computing (8 July 2008)
three-tier
3-tier
n-tier
<architecture> 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)