From The Free On-line Dictionary of Computing (30 December 2018) :
three-tier
3-tier
n-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)