A dominant trait in genetics refers to a characteristic that is expressed in an organism even when only one copy of the allele is present. Dominant alleles mask the effects of recessive alleles in a heterozygous genotype, leading to a phenotype that displays the dominant trait. For example, if a trait for brown eyes is dominant over blue eyes, a person with one brown eye allele will have brown eyes. Understanding dominant traits is crucial for predicting inheritance patterns in genetic studies.