Cyber-helmet

Cyber-helmet was a term used, by the Sixth Doctor at least, to refer to the head of a Cyberman. He kept one such Cyber-helmet in his C chest. (AUDIO: The Carrionite Curse)

In the case of the Cybermen from Pete's World, the brain their helmets contained was the sole organic component that remained in a Cyberman. (TV: Rise of the Cybermen/The Age of Steel)