An Inter-Set Rest Interval is a rest period between resistance exercise sets. A set is a number of exercises, i.e. 2, 6, 12, or any arbitrary number. Inter means between, and intra means within, so an intra-rest interval would be held between the exercises of the set, and an inter-rest interval is held after the completion of the set.
It should be known that studies have shown that for hypertrophy there is no longer a set amount of rest that's optimal, however, with any form of training the required rest should always be long enough to be able to perform the exercise perfectly, and in some cases to perform the exercise with or at the same intensity.
References:
- The effects of short versus long inter-set rest intervals in resistance training on measures of muscle hypertrophy: A systematic review
- The Effect of Inter-Set Rest Intervals on Resistance Exercise-Induced Muscle Hypertrophy
- Volume Load Rather Than Resting Interval Influences Muscle Hypertrophy During High-Intensity Resistance Training