Longman Dictionary of Contemporary English
Wiktionary
n. a school period in which a student has free time, i.e., is not enrolled in a class.
Wikipedia
Usage examples of "free period".
The best recent evidence suggests that Queen Maud Land, and the neighbouring regions shown on the map, passed through a long ice-free period which may not have come completely to an end until about six thousand years ago.
During a free period he showed the queer image to several professors, all of whom were intensely interested, though none of them could shed any light upon its nature or origin.
He usually enjoyed the free period between assignments, but this time Bill felt restless, stir-crazy, almost claustrophobic.
It had been a sad thing to read some of the comments made by females during the free period.
She took one low-grav step across to the hatch lock, let herself through, stopped just inside the shop to study what Tim and Bickel had accomplished since her last free period.
But at least arriving late for a free period is a less heinous crime than missing a lecture.