Visual Modeling with Rational Rose 2002 and UML (3rd Edition)

An object is a representation of an entity, either real-world or conceptual. An object can represent something concrete, such as Joe's truck or my computer, or a concept such as a chemical process, a bank transaction, a purchase order, Mary's credit history, or an interest rate.

An object is a concept, abstraction, or thing with well-defined boundaries and meaning for an application. Each object in a system has three characteristics: state, behavior, and identity.