Sigmoid function
The terms sigmoid function or sigmoid curve refer both to a special case of the logistic function and, more generally, are used to refer to of any real function of a real variable with a sigmoid ("S"-shaped) graph: i.e. a differentiable function whose first derivative is non-negative and has a single local maximum at a finite argument. Besides the standard function above and its examples of sigmoid functions (in this general sense) are the ordinary arc-tangent, the hyperbolic tangent, and the error function.
