Columbia points
Columbia points is the university's cutesy name for credits. You get 3 of them for an average class, although a single course can be anywhere from 0.5-9.0 points. A point roughly corresponds to one hour of instructional time per week.
Different degrees require different numbers of points from different parts of the university, and some students have to pay tuition per point. This has the odd effect of providing an economic disincentive to study more.
By contrast, Columbia College and SEAS undergraduate students pay tuition per semester, irrespective of the number of credits taken in the semester. Some overachieving undergraduates will petition to go over the maximum number of allowed points, which is 22 per semester for Columbia College and 21 for SEAS. A petition must pass through the Center for Student Advising. Students with semester courseloads in the low thirties, while ridiculous, are not unheard of.
It is also possible to petition to go under the minimum courseload for various reasons, but the definition of this threshold varies by program.