A change in allele frequency due to random events is called?

Genetic drift - A variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce.

Genetic drift (also known as allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene variant (allele) in a population due to random sampling of organisms ... A population's allele frequency is the fraction of the copies of one gene that share a particular form.