What does this mathematical symbol mean in the picture?

"γ" is the third Greek letter, pronounced as "gamma" and lowercase as "γ".

When used as a symbol of a mathematical function, it refers to the Gamma function, also called Euler's second integral, which is a function in which the factorial function is expanded on real numbers and complex numbers.

The gamma function in the real number field is defined as:

The gamma function in the complex domain is defined as: