Approximation refers to an estimate or a close approximation of something. It is a calculation that gives an approximate value, rather than an exact one, for a given quantity. In other words, it's a way of describing how closely a result matches the true value, with perfect being 100% and good being around 95%. Approximation can be used in various fields such as mathematics, physics or engineering to make calculations more efficient by reducing the number of digits that need to be calculated. It is often used when an exact answer would take too long to compute or when a high level of accuracy is not required.