First tier business schools are top ranked business schools. Although the term first tier isn't used anymore by most ranking organizations, it is a term that resonates among people who read the rankings.
The term "first tier business school" is interchangeable with the term "top business school." In other words, first tier business schools are the "best" business schools."
At one time, any school that was ranked among the top 50 schools by a ranking organization was considered a top tier business school. Nowadays, the term is used to describe any school that is ranked within the top 20, 30, or 50 business schools.

