Euler Constant was discovered by a genius mathematician in 1735. It’s used in various analytical formulas and number theory. Math scientists still argue if this constant is a rational, algebraic or transcendental number. It equals approximately 0,577. I doubt that mathematicians will forgive us for such an imprecise description, since after the comma there come thousands of signs, which we’re not going to reproduce.
 
Anyway, to talk about Euler Constant, you can use this symbol [U:2107] from Unicode. The value is often expressed with "gamma" – [U:1D6C4] or “C” – [U:0063].