# Back propagation cost function formula - what does $\sigma$ mean?

I'm a beginner in ML, and have a question on back propagation cost function.

I'm reading the material on this link. In the below screen, since $$a$$ is the neuron's output, then why $$a = \sigma(z)$$ instead of $$a = z$$ directly?

What does $$\sigma$$ mean exactly here?

• Sigma is called an activation function. Its used to add nonlinearity to the network. – Matthew Drury Sep 3 '17 at 13:04
• @MatthewDrury Thanks a lot Matthew! Yea, I missed the part for activation function. – Bon Sep 3 '17 at 13:25