### Summary

The Rectified Linear Unit, often shortened as ReLU, is a simple but widely used activation function in neural networks. It greatly helps the network learn complex patterns by introducing non-linearity into the model.

### ELI5

Imagine a group of kids are racing and the rule is that you can only move forward if you have a green flag, but if you get a red flag, you have to stop. Now, think of ReLU as that rule. It helps a computer deciding when to move information forward, and when to stop it.

### In-depth explanation

The Rectified Linear Unit, better known as ReLU, is a type of activation function that’s employed in artificial neural networks. The primary function of an activation function is to transform the input signal to an output signal, effectively deciding whether the neurode should fire often based on the weighted sum of inputs.

ReLU is expressed mathematically as `f(x)=max(0,x)`. Intuitively this means, for any input value `x`, if `x` is less than 0, the function returns 0, and if `x` is greater than or equal to 0, the function returns `x` itself.

In coding terms, this is often written compactly in Python as:

def relu(x): return max(0, x)

ReLU’s simplicity stands among its key advantages. Compared to other activation functions, ReLU involves less computational overhead, owing to its straightforward operation. During the backpropagation process, its gradient is either 0 or 1, which is computationally efficient for gradient descent optimization algorithms.

Moreover, ReLU helps mitigate the vanishing gradient problem that’s prevalent in deep learning models. However, it’s not without shortcomings; one common problem is the “dead ReLU” situation, where neurons essentially become inactive and only output 0 for any input.

Variants of ReLU, such as Leaky ReLU and Parametric ReLU, have been proposed to address its shortcomings while KathNets and Mish attempt to improve overall performance.

ReLU function is widely used in Convolutional Neural Networks (CNNs) and Deep Learning models, where it can help the network learn highly complex and non-linear relationships.

Activation Function, Neuron, Neural Network, Convolutional Neural Network (CNN),s, Backpropagation, Gradient Descent, Vanishing Gradient Problem, Leaky ReLU, Parametric ReLU, KathNets, Mish Activation Function