N 1893, Charles Eliot, president of Harvard, introduced to the National Education Association a novel concept: the credit hour. Roughly equivalent to one hour of lecture time a week for a 12- to 14-week semester, it became the basic unit of a college education, and the standard measure for transferring work between institutions. To be accredited, universities have had to base curriculums on credit hours and years of study.
via Degrees Based on What You Can Do, Not How Long You Went – NYTimes.com.