Difference between three tier vs. n-tier

Question

I just came across the following sentence:

As the industry has moved from a three tier model to n-tier models, the object relational impedance mismatch

Answer 1

Hey I can't even get a definition of 3 tier. Sometimes they discount javascript on the client and sometimes javascript on the client and the client web browser are considered another tier. So an ASP page that talks to a database can be 3 tier if you assume database = tier 3, web server = tier 2, client web browser = tier 1. And other times web server = tier 1, middleware = tier 2, database = tier 3. It really depends who is writing the definition/book.

In general n-tier seems to refer splitting the middleware layer up more. But aside from that I don't see consistent definitions.