"The present paper makes use of the Stone-Weierstrass Theorem and the cosine squasher of Gallant and White to establish that standard multilayer feedforward network architectures using abritrary squashing functions can approximate virtually any function of interest to any desired degree of accuracy, provided sufficently many hidden units are available." (Hornik et al., 1989, Neural Networks)
A squashing function is for example a nonlinear activation function that maps to [0,1] like the sigmoid activation function.
