Valiant was a name given to a large, Earth-bound, flying aircraft carrier used by UNIT during the early 21st century. (TV: The Sound of Drums, The Poison Sky, The Stolen Earth)
History
"Harold Saxon" said that he designed every detail of the Valiant, and so it must have been built, or at least completed, within the 18 months that the Master was working as Harold Saxon. It was used for the first human contact with the Toclafane. The Tenth Doctor, Martha Jones and Jack Harkness teleported onboard using Jack's vortex manipulator at the coordinates 58.2 north by 10.02 east. (TV: The Sound of Drums)
In the following year in an erased timeline, the Valiant was the base of operations for the Master. Martha's family (apart from Leo) and Captain Jack Harkness were imprisoned on the ship, as was the Doctor. It also held the paradox machine created from the Doctor's TARDIS. When the timeline was erased, the Valiant was the only point in space and time not to revert, instead moving directly through time, due to it being the "eye of the storm", that is, of the paradox machine. (TV: Last of the Time Lords)
During the Sontaran invasion of Earth in 2009, Colonel Mace called in the Valiant, which was referred to by the call-sign "Hawk Major". The ship's powerful turbines were used to clear the Sontarans' poisonous fog-like gas from the ATMOS factory and the Jathaa sunglider laser fitted to the ship was used to blast apart the factory, killing many Sontarans. (TV: The Poison Sky)
The Valiant was targeted when the New Dalek Empire attacked Earth following the planet's transportation to the Medusa Cascade. A massive force of Daleks swarmed the ship, taking out its shields. The order was given to abandon ship, and Jack Harkness reported that the Valiant was "down". (TV: The Stolen Earth)
Rusty the Dalek remembered the Daleks attacking the Valiant when the Twelfth Doctor and Clara Oswald attempted to reopen his suppressed memories. (TV: Into the Dalek)
At some point following the Dalek invasion, the Valiant was apparently salvaged and repaired, or replaced by another ship with the same name. The Twelfth Doctor asked whether he was being taken to the Valiant (mistakenly referring to the craft as "Cloudbase") during the Missy-initiated Cyberman attack on Earth, but Kate Stewart told him that doing so would be too obvious and too easy of a target. He was instead taken to an aircraft designated as Boat One. (TV: Death in Heaven)
Behind the scenes
According to non-narrative information in DWBIT 59, when the Earth was moved to the Medusa Cascade the Valiant took heavy damage from the Daleks. When it was forced over heavily populated areas, the decision was taken to land on empty farmland, avoiding a further loss of life. However, the attack still continued.