A test cross is an experimental cross of an individual organism of dominant phenotype but unknown genotype and an organism with a homozygous recessive genotype (and phenotype). It determines whether the individual with dominant phenotype is homozygous dominant or heterozygous dominant, by determining its genotype.
Related Theory
The purpose of the test cross is to determine the genetic makeup of the dominant organism. Mendel wanted to do this so that he could be sure he was working with a dominant organism which was homozygous, or contained only dominant alleles. However, the phenotype alone doesn't not tell you the genotype of an organism.