The phenomenon in which  a gene codes for a single character has more than two different alleles, which leads to expression of more than two different traits is referred to as

Alleles are alternate forms of a gene. Each gene can have a minimum of two alleles. Multiple allelism is a condition in which each gene has more than two alleles.

In general, a diploid organism has two copies of alleles per gene. Since these alleles are alternate forms of a gene, their location is on the same locus of homologous chromosomes.

In multiple alleles, each gene may have more than two alleles and, as a result, the traits are also more than two.

Ex: ABO blood grouping.