negatives (or eccentrics) is the phase of a rep where the muslce is lengthening against the weight. for example, in a curl, when the dumbell is moving toward the ground.
it can also refer to sets where the concentric portion of the rep is in some way "helped." this could be with a spotter or by using both limbs to move the wieght up and then doing the negative with justone are at a time.
generally you are able to do more weight with the negative, and its very important for hypertrophy.
