Traits are specific characteristics that vary among individuals of a species of organism. A trait must be measurable. The measuring unit for traits on the genetic level are genes. While the term phenotype may be used interchangeably with trait, it is not an accurate usage, for phenotype refers to the expression of the trait, not the trait itself. For example, eye color is a trait in humans, and the colors blue, green, brown, or hazel are four phenotypes of the eye color trait.