"Heterosis, broadly speaking, refers to any hybrid trait falling outside the range of parental variation". ie when a hybrid has traits that are different to those of its parents.

further on :
"Similarly, the hybrid produced by a lion and a tigress (known as a "liger") is usually larger and stronger than either of its parents, whereas the reciprocal cross produces a hybrid (the "tigon") that tends to be smaller than either of its parents. The former cross, then, is an example of positive heterosis, in which the hybrid exceeds the range of variation exhibited by its parents, while the latter is an example of negative heterosis, in which the hybrid falls below the range of parental variation with respect to a given trait."

ie positive heterosis means the tendency of a hybrid organism to have traits superior to those of either parent