The phrase "multiple alleles" in biology refers to a series with at least three allelic or alternative forms of a particular gene, with only two that are able to exist in a diploid individual. There are two alleles present in an individual: a paternal and a maternal allele.
Gregor Mendel, the father of genetics, proposed that only two alleles exist for each gene, with the dominant form expressing its phenotype within a heterozygote. However, it is possible for more than two different alleles to exist inside a population. Use of the term "multiple alleles" is occasionally confusing. This does not suggest that a specific gene has three or more alleles. No individual has the capacity for more than two alleles, one paternal and one maternal, even if more alleles are a part of the population.
One example of a gene having multiple alleles is the canine C series. The gene "C" refers to a dog with the color series expressed. "c" is an albino, with pale nose and eyes. "cb" refers to a gray, pale dog, while "cd" refers to a dark-eyed white dog. "c-ch" would be a dog with a chinchilla pattern. The C allele (the color series expressed) is dominant to all of the other alleles, while "c-ch" is dominant to all of the others ("cd," "cb," and "c"). The albino gene is recessive to all the rest.