From Citizendium, the Citizens' Compendium
Definition (general) : A statement of what something is. It commonly has the form [Name] is a [Category] that [Point of Difference]. This form enables a person to learn new knowledge by building on what's already understood. For example, the definition of a horse might be as follows: A horse (name) is an animal (known category) that has four legs and mane and runs fast (point of difference -- how a horse is different from other animals). So a person who knows what an animal is, but doesn't understand what a horse is, can add to his or her knowledge in this way. In a scientific sense, a definition is a description which is valid under precisely stated conditions.