Helmet

A helmet was a form of protective gear worn to protect the head.

Just after arriving in Northumbria in 1066, Vicki Pallister found a Viking helmet which she shown to the First Doctor. Though the First Doctor attempted to use this as proof for Steven Taylor that his TARDIS could travel through time, Steven voiced his reservations, to which the Doctor sarcastically asked him if he thought it was a space helmet for a cow. Steven speculated that it was part of a costume, a toy left behind by a child. (TV: The Time Meddler)

The heads of Cybermen were known, at least by the Sixth Doctor, as Cyber-helmets. (AUDIO: The Carrionite Curse)

Sontaran troopers typically wore helmets. Certain Sontarans, however, could chose to go without their helmet in what was refered to being "open-skinned". (TV: The Sontaran Stratagem)