Sigmoid

The sigmoid function is a mathematical function that produces a sigmoidal curve—a characteristic curve known for its S shape. This is the earliest and often used activation function. This squashes the input to any value between 0 and 1 and makes the model logistic in nature. This function refers to a special case of logistic function, defined by the following formula:

The following screenshot shows a sigmoid curve with an S shape:

The sigmoid function is differentiable; this means that we can find the slope of the sigmoid curve at any point in its existence interval. This is a monotone function with a nonmonotonic derivative. The logistic sigmoid function is a sigmoid function as well as a softmax function, which is a more generalized logistic activation function that's used for multiclass classification.