What is a symbol representing a value that does not change?

I assume you are talking about constant values in expressions. Any letter (including Greek ones) can be used to represent a constatnt, but the most common one is k.

There are, however, some constants which are associated with other symbols:

Ratio of circumference of circle to diameter = pi

Speed of light in vacuum: c

Gravitational attraction: G (not to be confused with g, in the context of the earth's gravity)

Avogardo's constant: NA