Beefalo are a fertile

Fertility

Fertility is the natural capability of producing offsprings. As a measure, "fertility rate" is the number of children born per couple, person or population. Fertility differs from fecundity, which is defined as the potential for reproduction...

 hybrid offspring of domestic cattle

Cattle

Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...

, Bos taurus, and the American bison

American Bison

The American bison , also commonly known as the American buffalo, is a North American species of bison that once roamed the grasslands of North America in massive herds...

, Bison bison (generally called buffalo in the US). The breed was created to combine the characteristics of both animals with a view towards beef production.

Beefalo are primarily cattle in genetics and appearance, with the breed association defining a full beefalo as one with ⅜ (37.5%) bison genetics, while animals with higher percentages of bison genetics are called "bison hybrids".
Accidental crosses were noticed as far back as 1749 in the southern English colonies of North America.