Sigmoid Function

The name Sigmoidal comes from the Greek letter Sigma, and when graphed, appears as a sloping "S" across the Y-axis. A sigmoidal function is a type of logistic function and purely refers to any function that retains the "S" shape, such as tanh(x). Where a traditional sigmoidal function exists between 0 and 1, tanh(x) follows a similar shape, but exists between 1 and -1. On its own, a sigmoidal function is also differentiable, meaning we can find the slope of the sigmoid curve, at any two points.