The expression "\Gamma(x+y)" refers to the Gamma function, which is a mathematical function that extends the concept of factorials to non-integer values. For positive integers, the Gamma function is defined as \Gamma(n) = (n-1)!. For example, \Gamma(5) = 4! = 24. The function is defined for all complex numbers except for non-positive integers. In the context of \Gamma(x+y), it represents the Gamma function evaluated at the sum of two variables, x and y. This function is useful in various fields, including statistics and probability, particularly in defining distributions like the Gamma distribution and Beta distribution.