A helmet was a form of protective gear worn to protect the head. Warriors of numerous races wore helmets, such as humans (TV: The Time Meddler, et al.) Sontarans, (TV: The Time Warrior) Sycorax, (TV: The Christmas Invasion) Judoon (TV: Smith and Jones) and the Mire. (TV: The Girl Who Died)
History
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) In the case of those from Pete's World, the brain they contained was the sole organic component that remained in a Cyberman. (TV: Rise of the Cybermen/The Age of Steel)
Sontaran troopers typically wore helmets. Certain Sontarans, however, could chose to go without their helmet in what was referred to being "open-skinned". (TV: The Sontaran Stratagem) The helmets of Sontarans of the Special Assault Squad, such as Kaagh, could be automatically retracted into their armour at will. (TV: The Last Sontaran)
Sycorax warriors wore helmets known as skull helmets. (TV: The Christmas Invasion, PROSE: The Final Darkness)
A the Twelfth Doctor's request, Ashildr used a stolen Mire helmet to project a false image which scared them into fleeing from Earth, an act which resulted in her death. (TV: The Girl Who Died) Another such helmet was captured and held within UNIT's Black Archive, (TV: The Zygon Inversion) while yet another was found inside the Matrix on Gallifrey. (TV: Hell Bent)