A lifeform, also written as life form or life-form, was a generic term for a living being. According to the Eleventh Doctor, lifeforms were just sensors developed by his universe across its surface to "become aware of itself", and had little to none individual existence. (PROSE: The Day of the Doctor)
A number of silicon-based lifeforms were encountered by the Doctor, including the Kastrians, (TV: The Hand of Fear) Ogri (TV: The Stones of Blood) and Plutonians. (PROSE: GodEngine)
A messenger told the Fourth Doctor that the Time Lords had foreseen a time where the Daleks had destroyed all other lifeforms; it was for this reason that the Fourth Doctor was assigned the mission of interfering with the Daleks' development. When the Doctor confronted Davros about the evil of the Daleks, the Dalek creator insisted that, should the Daleks become the only lifeform in the universe, peace would reign. (TV: Genesis of the Daleks) The Daleks inherited this belief, using it as justification and rationalisation for their crusade against the Thals. (AUDIO: The Lights of Skaro, Return to Skaro)
In the 29th century, Taren Capel tried to start a robot revolution, as he believed robots to be a superior lifeform and wanted to liberate them. (TV: The Robots of Death)
CAIN was an artificial lifeform on the Myriad, with a fungoid brain. (AUDIO: The Pyralis Effect)
Strange icarons had no harmful effects on organic lifeforms, but interfered with starship jump engines. (PROSE: GodEngine) Conversely, normal icarons "preferred" to be in hyperspace and caused paranoia and psychotic behaviour in carbon-based lifeforms. (PROSE: Original Sin)
Gabrielideans were a liquid-based lifeform. One of their soldiers fighting for the Time Lords in their War against the enemy was captured and turned into an idea-based lifeform known as the Shift. (PROSE: Alien Bodies)
Historians who had been allowed access to the "Monster Vaults" of the databanks in the Doctor's TARDIS observed that every part of an Abzorbaloff's body "from head to toe" was in a constant state of suction, thus consuming any living thing upon contact. (PROSE: The Monster Vault, TV: Love & Monsters)
When the Tenth Doctor and Donna Noble visited the Library, its computers confirmed they were the only humanoid life present, but then told them there were over "a million million lifeforms" in the Library. (TV: Silence in the Library)