N-tier application architecture provides a model for developers to create a flexible and reusable application.
By breaking up an application into tiers, developers only have to modify or add a specific layer, rather than have to rewrite the entire application over, if they decide to change technologies or scale up.
In the term “N-tier,” “N” implies any number — like 2-tier, or 4-tier; basically, any number of distinct tiers used in the architecture.