What is the definition for a dominant trait?

Dominant traits are those that are expressed or seen when a heterozygous genotype is present. Heterozygous just means having two different alleles for a gene. One is usually dominant while one is recessive. Remember, recessive alleles are masked by dominant alleles. In humans, dark hair is a dominant trait; if one parent contributes a gene for dark hair and the other contributes a gene for light hair, the child will have dark hair.