What is the name of Mendel's law that describes allele separation?

The Mendelian law that describes allele separation is the Law of Segregation.

The law states that an organism’s two alleles for a gene separate during gamete formation, so each egg or pollen grain receives only one allele. At fertilization, the offspring receives one allele from each parent, restoring the pair. Mendel developed this idea from pea crosses in which a recessive trait disappeared in one generation and returned in the next.

For example, a heterozygous pea plant carrying one purple-flower allele and one white-flower allele produces gametes carrying either allele. When two such plants are crossed, the expected genotypes are 1:2:1: one homozygous dominant, two heterozygous, and one homozygous recessive. Because purple is dominant in Mendel’s example, the visible flower-color ratio is about 3:1.

The law is sometimes called Mendel’s First Law, although terminology varies. It should not be confused with the Law of Independent Assortment, which concerns how allele pairs for different traits are distributed.