American Institutions of Higher Educations use academic terms (or simply "term") as a portion of an academic year, the time during which an educational institution holds classes. The most common term is the semester system, followed by the quarter, 4-1.4, and the trimester.
- A semester system divides the academic year into two terms of equal length, with attendance required in both semesters to total 32 to 36 weeks of instruction. There is often an optional summer session half as long as a full semester.
- A quarter system divides the academic year into four terms, one per season, with attendance required in three-quarters per year to total 32 to 36 weeks of instruction. A "quarter" system treats the summer term on an equal footing with the other terms. It divides the academic year into four quarters, each of which is usually 12 weeks long. Three of the four quarters (Autumn/Fall, Winter, and Spring, operating from September until June or from August until May) are thus equivalent to two 18-week semesters. A "quarter" system treats the summer term on an equal footing with the other terms. It divides the academic year into four quarters, each of which is usually 12 weeks long. Three of the four quarters (Autumn/Fall, Winter, and Spring, operating from September until June or from August until May) are thus equivalent to two 18-week semesters. Thus, when American academic universities convert academic credits between the semester/trimester and quarter systems, 36 quarter hours convert to 24 semester hours (⅔ conversion factor) while 36 semester hours convert to 54 quarter hours (3/2 conversion factor).
In practice, the average quarter-long course is four or five units and the average semester course is three units, so a full-time student graduating in four years would take five courses per semester and three or four courses per quarter.
- The 4-1-4 calendar system is a variation of the semester system with a January term between the Fall and Spring terms. A 4-1-4 or 4-4-1 academic calendar incorporates two terms that last approximately 14 weeks, with the addition of a smaller one month term that falls either in January or May. The January term consists of only one class for the entire period. This allows classes to meet at outside the regular time schedules or even travel off campus as part of the studies such as, independent study, study abroad, internships, activities, or focus on one or two classes.
Some colleges follow a modified 4-1-4 academic calendar, with two 13-week semesters, one week of exams in each semester and a four-week semester in May.
- A trimester system divides the academic year into three terms of 14 to 16 weeks each, with attendance required in two trimesters per year, to total 28 to 32 weeks of instruction. The trimester evolved from the semester system in the 1960s and, thus, is compatible with it (i.e. it is simply a semester shortened from 16–18 weeks to 14–16 weeks). The spring-summer trimester may be a full trimester or may be divided into distinct spring and summer half-trimester or shorter sessions, in which classes meet double-time (or greater) to provide the same instruction that would be received in a full trimester.