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Definition: in mathematics, a fixed quantity or one that does not change its value in relation to variables. For example, in the algebraic expression y 2 = 5x - 3, the numbers 3 and 5 are constants. In physics, certain quantities are regarded as universal constants, such as the speed of light in a vacuum.
Source: Helicon Publishing. Hutchinson Pocket Dictionary of Maths. Abingdon, Oxfordshire, GBR: Helicon Publishing, 2005. p 43.