Activity: A duration of time of specified length (e.g., a service time or inter arrival time), which is known when it begins (although it may be defined in terms of statistical distribution).

System: The system is defined as a group of objects that are joined together in some regular interaction or interdependence towards the accomplishment of some purpose. OR It is defined as an aggregation of objects that are joined together in some regular fashion for the accomplishment of some purpose. OR it is a collection of entities (e.g., people and machines) that interact together over a time to accomplish one or more goals. Example: Production system manufacturing automobiles. In this system machines, component parts and workers operate jointly along an assembly line to produce a high quality vehicle.

Simulation: Simulation is the imitation of the operation of real world process or system over time. Example: Simulation of inventory system, simulation of queuing system.

Delay: Duration of time of unspecified indefinite length, which is not known until it ends. Example: a customer’s delay in last in first out waiting line which, when it begins, depends on future arrivals.

Model: A representation of a system, usually containing structural, logical, or mathematical relationships that describes a system in terms of state, entities and their attributes, sets, processes, events, activities and delay. A model contains only those components that are relevant to the study of system. The model should be complex enough to answer the questions raised and should be simple enough to make the understanding easy. Example: Model of a building is developed before the construction.