x1 is the Dirac delta function (generalized), defined on structure x2 (contextless default is probably the field of real numbers), yielded by family of distributions x3 (contextless default is probably Gaussians centered at 0 and which enclose unit area)
This generalized function evaluates to zero (0) everywhere except at 0 (in the domain), at which it evaluates to an infinity (∞) sufficient(ly large) for the purpose of integration to exactly equal one (1) whenever the integral interval properly contains 0 (in the domain). x2 determines what 0, 1, and ∞ mean. Properly, more than a set should be specified; the domain and codomain are determined thereby.