Heterosis, also known as hybrid vigor or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. It refers to the phenomenon that progeny of diverse varieties of a species or crosses between species exhibit greater biomass, speed of development, reproductive traits, longevity, calf survivability, pounds of calf weaned, and younger age at puberty. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions of its parents. Heterosis is often discussed as the opposite of inbreeding depression, which occurs when related parents have children with traits that negatively influence their fitness largely due to homozygosity. Inbreeding depression can be overcome by outcrossing, which should result in heterosis.

Heterosis is not limited to the effects that can be explained by heterozygosity in Mendelian inheritance. The amount of heterosis that is realized for a particular trait is inversely related to the heritability of the trait. Traits that are lowly heritable, such as reproductive traits, generally benefit from heterosis the most. End-product traits that benefit from heritability in the moderate to high range benefit less from heterosis. There are three main types of heterosis: individual, maternal, and paternal.

In summary, heterosis is the improved or increased function of any biological quality in a hybrid offspring. It is often discussed as the opposite of inbreeding depression and can be overcome by outcrossing. Heterosis comes in three different forms: individual, maternal, and paternal. It is most beneficial for lowly heritable traits such as reproductive traits.