In biology, a trait is a specific characteristic or attribute of an organism that can be described or measured. Traits can be determined by genes, environmental factors, or a combination of both. They can be physical, such as hair color, leaf shape, or muscle definition, or behavioral, such as bird nesting. Traits can be qualitative, such as eye color, or quantitative, such as height or blood pressure. A given trait is part of an individuals overall phenotype.

Scientists often study twins to understand how genes and the environment work together to affect traits. They compare traits in identical twins, who have identical DNA, and fraternal twins, who share half their DNA, just like any siblings. If a characteristic appears more frequently in identical twin pairs than in fraternal twin pairs, then it has an inherited component.

In summary, traits are characteristics or attributes of an organism that can be expressed by genes and/or influenced by the environment. They can be physical or behavioral, qualitative or quantitative, and are part of an individuals overall phenotype.