Simulation is the imitation of the operation of a real-world process or system over time. The act of simulating something first requires that a model be developed; this model represents the key characteristics or behaviors/functions of the selected physical or abstract system or process. The model represents the system itself, whereas the simulation represents the operation of the system over time.
Simulation is used in many contexts, such as simulation of technology for performance optimization, safety engineering, testing, training, education, and video games. Often, computer experiments are used to study simulation models. Simulation is also used with scientific modelling of natural systems or human systems to gain insight into their functioning.〔In the words of the (Simulation article ) in Encyclopedia of Computer Science, "designing a model of a real or imagined system and conducting experiments with that model".〕 Simulation can be used to show the eventual real effects of alternative conditions and courses of action. Simulation is also used when the real system cannot be engaged, because it may not be accessible, or it may be dangerous or unacceptable to engage, or it is being designed but not yet built, or it may simply not exist.
Key issues in simulation include acquisition of valid source information about the relevant selection of key characteristics and behaviours, the use of simplifying approximations and assumptions within the simulation, and fidelity and validity of the simulation outcomes.
==Classification and terminology==
Historically, simulations used in different fields developed largely independently, but 20th century studies of systems theory and cybernetics combined with spreading use of computers across all those fields have led to some unification and a more systematic view of the concept.
''Physical simulation'' refers to simulation in which physical objects are substituted for the real thing (some circles〔For example in computer graphics (SIGGRAPH 2007 | For Attendees | Papers ) (Doc:Tutorials/Physics/BSoD - BlenderWiki ).〕 use the term for computer simulations modelling selected laws of physics, but this article doesn't). These physical objects are often chosen because they are smaller or cheaper than the actual object or system.
''Interactive simulation'' is a special kind of physical simulation, often referred to as a ''human in the loop'' simulation, in which physical simulations include human operators, such as in a flight simulator or a driving simulator.
Human in the loop simulations can include a computer simulation as a so-called ''synthetic environment''.〔Thales defines synthetic environment as "the counterpart to simulated models of sensors, platforms and other active objects" for "the simulation of the external factors that affect them"() while other vendors use the term for more visual, virtual reality-style simulators ().〕
''Simulation in failure analysis'' refers to simulation in which we create environment/conditions to identify the cause of equipment failure. This was the best and fastest method to identify the failure cause.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』