Constant

A constant is a value that does not change its value during a particular process (in contrast to a variable, the value of which can change). Examples of constants are pi, coefficients of polynomials, and temperature during an isothermal process.[Клепко В., Голець В.]
Defined constants are important invariants, generally considered to be exact dimensionless numbers in mathematics and approximate (and often but not always) measurable quantities in experimental science.
In mathematics, undefined constants are also used, which can have an arbitrary value without changing the essential properties of the mathematical object.
Like any mathematical object, constants must be strictly defined. The concept of a constant in mathematics depends fundamentally on such a number. The most basic numbers are the natural numbers determined using set theory. [Шмиг Р., Боярчук М., Добрянський В., Барабаш В.]

Sources:

Клепко В., Голець В. (2009). Поняття сталої величини. Київ: Центр учбової літератури.

Шмиг Р., Боярчук М., Добрянський В., Барабаш В. (2010). Термінологічний словник з будівництва та архітектури. Львів: Львівська політехніка.