Slater-type orbital. Basis function with an exponential radial function, i.e., exp(- zeta r). Also used to denote a fit to such a function using other functions, such as gaussians. For example, STO-3G is an MBS that uses 3 gaussians to fit an exponential. Exponentials are probably better basis functions than gaussians, but are so much more difficult computationally that they were abandoned by most people a long time ago.