The term backcross means that an F₁ or F₁' individual is mated to one of its parents (P₁ or P₂) or to another individual that is genetically identical to one of its parents.  The term backcross may be used as a verb (describing the process of setting up the required mating) or as a noun to describe the mating or to describe the progeny that result from such a hybrid cross.

The B₂ backcross is a cross of an F₁ or F₁' individual to a P₂  individual.  The cross of  an F₁ or F₁' individual to a P₁ individual is the B₁ cross.   The B₂ backcross may also be called the test cross.

The expected results of such a cross will change according to the dominance of the alleles involved and whether the genes are autosomal and sex-linked.