Invisibility

Invisibility was the state of being invisible. Many species were invisible.

Biology
Visians were an invisible species of predators native to Mira. The First Doctor, Steven Taylor and Sara Kingdom encountered the "vicious" Visians after being transported to Mira. (TV: The Daleks' Master Plan)

The Refusians once had physical form. A giant solar flare caused the Refusians to lose these bodies, and as a result, they no longer had a recognisable "being". The Refusians were not only invisible to other races, they were also invisible to one another; they could only sense another Refusian's presence. (TV: The Ark)

Spiridons, also known as Zalerians, were the invisible humanoid natives of Spiridon. Anti-reflecting light waves made the Spiridons naturally invisible at all times to all but the eye-plants. After death, a Spiridon would become completely visible. At one point, the Daleks occupied Spiridon, hoping to discover the secret of invisibility from the Spiridons. (TV: Planet of the Daleks)

The Krafayis were a species of invisible, space-faring predators. The Eleventh Doctor, Amy Pond and Vincent van Gogh encountered a Krafayis that had been blinded and stranded on Earth. (TV: Vincent and the Doctor)

Technology
The planet Phryne (but not its moon) was hidden behind an invisibility shield. After the Daleks discovered the moon, they were able to pierce the shield and invaded the planet. (COMIC: The Archives of Phryne)

Removing the visual stabiliser circuit from a TARDIS resulted in making it invisibile. (TV: The Invasion)

The Twelfth Doctor was provided with an invisibility watch. (TV: The Caretaker)