This is ζ(3) with the Riemann zeta function, a constant whose value is calculated with the infinite sum 1/13+1/23+1/33+1/43+... This constant is named after Roger Apéry, who proved it to be irrational. It is not known whether it's a transcendental number or not. This constant arises in a number of mathematical and physical problems.
Unlike ζ(2) and ζ(4), ζ(3) is not known to have a compact elementary expression.