Approximation

The replacement of mathematical objects by others which resemble them in certain respects

An approximation is a fact, object, or description similar to something else but which is not the same.
An approximation is a number, calculation, or position close to a correct number, time, or position but is not exact [Collins English Dictionary].

An approximation is a rough estimate or guess of something. When you write a budget for the month, you're making an approximation of how much you're going to spend. You can't be sure of the exact amount [Vocabulary.com].

Approximation - the act or process of drawing together.
Approximation - the quality or state of being close or near [Webster`s Dictionary].

Approximation - a guess of a number that is not exact but that is close [Cambridge Dictionary].

It is possible to study in this way numerical characteristics and qualitative properties of an object by reducing the problem to the study of simpler or more convenient objects (e.g. of objects with readily calculable characteristics or with properties which are known). Number theory deals with Diophantine approximations, in particular with approximations of irrational numbers by rational numbers. Approximations of curves, surfaces, spaces and mappings are studied in geometry and in topology. Certain branches of mathematics deal almost exclusively with approximations — e.g. the theory of approximation of functions and the theory of numerical methods in analysis.[Encyclopedia of Mathematics]

Sources:

Approximation. Collins English Dictionary. Retrieved from: https://www.collinsdictionary.com/dictionary/english/approximation

Approximation. Vocabulary.com - English Dictionary. Retrieved from: https://www.vocabulary.com/dictionary/approximation

Approximation. Webster`s Dictionary. Retrieved from: https://www.merriam-webster.com/dictionary/approximation

Approximation. Cambridge Dictionary. Retrieved from: https://dictionary.cambridge.org/dictionary/english/approximation

Approximation. BSE-3 (originator), Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Approximation&oldid=18313