In mathematics a radial basis function is a real-valued function φ {\textstyle \varphi } whose value depends only on the distance between the input and some fixed point, either the origin, so that φ = φ ^ {\textstyle \varphi ={\hat {\varphi }}}, or some other fixed point c {\textstyle \mathbf {c} }, called a center, so that φ = φ ^ {\textstyle \varphi ={\hat {\varphi }}}. Wikipedia