In general, a tier (pronounced TEE-er ; from the medieval French tire meaning rank, as in a line of soldiers) is a row or layer in a series of similarly arranged objects. In computer programming, the parts of a program can be distributed among several tiers, each located in a different computer in a network. Such a program is said to be tiered , multitier , or multitiered .
The 3-tier application model is probably the most common way of organizing a program in a network. N-tier applications (programs) are those that are tiered but the number of tiers isn't specified or may vary.