An academic term is the period in which a single set of classes is taken. Three classes are taken each term, for which grades are posted at term's end. Unlike many schools, WPI operates on a four-term system, with the terms labeled A through D and each lasting seven weeks, which corresponds to half a "standard" semester. As such, terms at WPI could accurately be referred to as "quarters", but almost nobody does this.
Between each WPI term is a break, which is usually a welcome opportunity to go home and forget about school for a few days. The breaks between A-B Terms and C-D Terms are 11 days each, and the break at Christmastime (between B-C Terms) is about four weeks long.
There is also a fifth term, E-term, which occurs during the summer months and is not considered part of the academic year.
For more information, see Time Keeping.