Noun
beta function (plural beta functions)
(mathematical analysis) A mathematical function that is also called the Euler integral of the first kind; it is a symmetric special function.
Considering the equation of the binomial coefficient n choose k described in terms of the factorial, replacing n with p + q and k with p, turning the RHS upside-down and replacing factorials with the Gamma function, then replacing (p + q) choose p with Β(p, q) yields Euler's equation for the Beta function.