Occam's razor is the principle enunciated by William of Ockham (English logician 1285–1349) that "entities must not be multiplied beyond necessity". This is particularly important in the description of the physical world, and in modeling. For example, the number of parameters and dimensionality of a model should be kept to a minimum.
In the modeling of air flow in meteorology, it is common to assume that air is incompressible, although it clearly is not.◊ The reasons for this are
the velocities and pressure differences involved are low and do not contribute greatly to the predictions, and
including compressibility would introduce sound waves making the problem hyperbolic and complicating numerical simulation (e.g., requiring that time-steps satisfy a Courant–Friedrichs–Levy conditions).