1. A conceptual, mathematical, or physical system obeying certain specified conditions, whose behavior is used to understand the physical system to which it is analogous in someway [22]. 2. A conceptual description and the associated mathematical representation of a system, subsystem, components, or condition that is used to predict chances from a baseline state as a function of internal and/or external stimuli and as a function of time and space [22]. 3. A simplified system bearing some physical similarity to a prototype [16].