Invisibility

Invisibility was the state of being invisible. According to the Twelfth Doctor, invisibility was a matter of reversing light waves. (TV: The Caretaker)

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

The Refusians once had physical form but 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; only being able to 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)

The Pied Piper had the ability to turn himself invisible. He challenged the First Doctor to catch him in his invisible state. The Doctor passed the test by using his echo-sounder to track him. (COMIC: Challenge of the Piper)

The Birdmen had the power of invisibility. (COMIC: The Invisible Invaders)

A glass avatar of Nardole once claimed he had "invisible" hair. (TV: Twice Upon a Time)

Technology
Anastas Thrax surrounded his space station with an invisible mist to hide it from Earth authorities. (COMIC: The Hijackers of Thrax)

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 invisible. (TV: The Invasion) The Eleventh Doctor used this feature (with some help from River) when visiting the Oval Office in 1969. (TV: The Impossible Astronaut)

The Twelfth Doctor had an invisibility watch. (TV: The Caretaker) He later recalled that he'd once invented an invisible watch, inviting Petronella Osgood to spot the obvious design flaw. (TV: The Zygon Inversion)

The Daleks later made their home planet Skaro invisible while Davros lured the Twelfth Doctor, Clara Oswald and to the rebuilt Dalek City. Initially thinking they were on a spaceship, Missy discovered the use of invisibility because the gravity was normal. (TV: The Magician's Apprentice)

In the far future, humanity made Earth (which they could pilot through space) invisible during their war against the Saurians, who greatly desired the secret of invisibility. (PROSE: Lords of the Galaxy)