AI Glossary
Backpropagation
Also called backprop
What does “Backpropagation” mean?
Backpropagation: The algorithm that works out how much each weight in a neural network contributed to the error. Starting from the output, it works backward through the layers using the chain rule from calculus, and it produces the gradients that gradient descent needs.

What is an example of “Backpropagation”?
Without backpropagation, training a network with billions of weights would be impractical, because computing each weight's effect separately would take far too long.