By "tier" I mean a place where code runs. For example, the client tier, the web app server tier, etc.
So tier splitting is about taking a class and making certain parts of it run in different tiers. For example, I want the "Save" method of my business object to run on the server where it can access the database. However, the rest of the time it needs to be
usable by the client tier to do business logic, validation, etc.