A neuron is the atomic unit of a neural network. This is sometimes inspired by biology; however, that's a topic for a different book. Neurons are typically arranged into layers. In this book, if I'm referring to a specific neuron, I'll use the notation  where l is the layer the neuron is in and k is the neuron number. As we will be using programming languages that observe 0th notation, my notation will also be 0th based.

At their core, most neurons are composed of two functions that work together: a linear function and an activation function. Let us take a high-level look at those two components.