The Ark was the name given by Dodo Chaplet to an enormous spacecraft. It was built circa 10,000,000 to carry the entire human, Monoid, animal and plant population of Earth, which was about to plunge into the Sun, to a new home on Refusis II. Because the journey would take 700 years, all but the Guardians were miniaturised and placed in suspended animation.
The First Doctor, Steven Taylor and Dodo visited the Ark twice, once at the beginning and once at the end of its journey. (TV: The Ark)
One of Clara Oswald's splinters wore the clothing of the Ark's Guardians. (COMIC: Blood and Ice)