A tier usually means physical separation at the machine level or, at a minimum, at the process level. A typical three-tier application architecture has the presentation layer code running on a client machine, the business logic and data access layers running on a middle-tier application server, and the database running on its own server as the third tier.
(Homonym: tear [crying]).
1. One of a set of rows placed one above or behind the other.
2. One of two or more layers one atop another.
3. One that ties.
1. To place or arrange in tiers
2. To rise in tiers