Define the term variation in the context of evolution?

Variation means difference. Any given organism has a variety of traits. It grows to a certain size, it has certain kinds of organs, limbs, or structures; it has teeth or shells or bones, and if it does they have various sizes and shapes; there are a great many such traits. Every trait can vary. One person grows to a height of five foot ten, another grows to six feet; that is a variation. One person has black hair, another has red hair; that is a variation. Every way in which one organism differs from another constitutes a variation. Of course, some variations are more significant than others. Some alter the course of evolution, and others don't.