Definition
Two exercises done back to back with little or no rest between them, then a rest before the next pair.
The useful kinds: antagonist pairs (bench press and row), where performance holds and the session gets shorter; same-muscle pairs for hypertrophy, where the second exercise finishes what the first started; and upper/lower pairs for conditioning.
Example: 4 rounds of 8 dumbbell presses and 8 cable rows, 75 seconds of rest after the pair. Sessions run 30 to 40% shorter with no loss on the lifts that matter.
Do not superset two heavy barbell lifts, and keep the second movement simple.