Model is an analogue of an object under cognition, which replaces the object in the cognition process. Conscious replacement of the reality with a model is called modelling. Further, when speaking about model, we mean Mathematical Model - abstraction of a real world problem or situation into a mathematical form. 

Mathematical modelling can be used for a number of different reasons. How well any particular objective is achieved depends on both the state of knowledge about a system and how well the modelling is done. Examples of the range of objectives are:

- Developing scientific understanding through quantitative expression of current knowledge of a system. As well as displaying what we know, this may also show up what we do not know.

- Test the effect of changes in a system.

- Aid in decision-making, including tactical decisions by managers and strategic decisions by planners.