A mathematical constant is a quantity whose value does not change; it is thus the opposite of a variable. Unlike physical constants, mathematical constants are defined independently of any physical measurement. Mathematical constants are typically elements of the field of real numbers or complex numbers. Mathematical constants that one can talk about are definable numbers (and almost always also computable).
Heraclitus, a pre-Socratic, is known for the phrase "The Only Thing That Is Constant Is Change.”