Simulation is an imitative representation of a process or system that could exist in the real world. In this broad sense, simulation can often be used interchangeably with model. Sometimes a clear distinction between the two terms is made, in which simulations require the use of models; the model represents key characteristics or behaviors of the selected system or process, whereas the simulation represents the evolution of the model over time. Another way to distinguish between the terms is to define simulation as experimentation with the help of a model. This definition includes time-independent simulations, using computers.

Simulation is used in many contexts, such as simulation of technology for performance tuning or optimizing, safety engineering, testing, training, education, and visualizations. Simulation is also used with scientific modelling of natural or human systems to gain insight into their functioning, as in economics directly related to manufacturing. Simulation can be used to show the eventual real effects of alternative conditions and courses of action. Simulation is used when the real system cannot be engaged as well, because it may not be accessible, or it may be dangerous or unacceptable to engage, or it is being designed but not built yet, or it may simply not exist.

Key issues in modeling and simulation include the acquisition of valid sources of information about the relevant selection of key characteristics and behaviors used to build the model, the use of simplifying approximations and assumptions within the model, and fidelity and validity of the simulation outcomes. Procedures and protocols for model verification and validation are ongoing fields of academic study, refinement, Research and Development (R&D) in simulation technology and practice, particularly in the area of computer simulating materials, product design, and manufacturing processes.

In a product development context, a computer simulation is an attempt to model a real-life or hypothetical situation to study on a computer how the product ends up working for a combination of a material, a design, and a manufacturing process. By changing variables in the simulation, predictions may be made about the behavior of the preset combination. It is a tool to virtually investigate the behavior of a product or a system under study.

Traditionally, the formal modeling of systems has been through mathematical models, which attempt to find analytical solutions enabling the prediction of the behavior of a system from a set of parameters and initial conditions. Computer simulation is often used as an adjunct to, or substitution for, modeling systems for which simple closed-form analytic solutions are not possible. There are many different types of computer simulations, the common feature they all share is the attempt to generate a sample of representative scenarios for a model in which a complete enumeration of all possible states would be prohibitive or impossible.

Several software packages exist in research circles or on the marketplace for running computer-based simulation modeling (e.g., Monte Carlo analysis, stochastic modeling, multimethod evaluation, FEA, CFD). Complementarily, process simulation is used for the design, development, analysis, and optimization of a technical process at processing plants, about chemical evolutions, related to environmental systems, power stations, complex manufacturing operations, biological processes, etc.

The development of models for a better representation of real processes is the core of further development of simulation software. Model development is done through the principles of chemical and physical engineering but also control engineering for the improvement of mathematical simulation techniques.