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A constant is a name for an object (usually a number, but the concept has wider applications) which does not change during the context of a logical or mathematical argument.

A constant can be considered as an operator which takes no operands.

A constant can also be considered as a Definition:Variable whose domain is a singleton.

Also see

Historical Note

The term constant, as opposed to a variable, was introduced by Gottfried Wilhelm von Leibniz.

Linguistic Note

The word constant can be used either as a noun:

Let $c$ be a constant between $0$ and $1$

or as an adjective:

Let $c$ be a constant real number between $0$ and $1$

Which is intended can usually be deduced from the context.