Alifeform (also renderedlife form orlife-form, and often known as anorganism) was an individual living being; a unit oflife. A greatdiversity of lifeforms existed. (TV:Silence in the Library^[+]Loading...["Silence in the Library (TV story)"]) However, according to theEleventh Doctor, all lifeforms were justsensors developed byhis universe across its surface to "becomeaware of itself", and had little to no individual existence. (PROSE: The Day of the Doctor^[+]Loading...["The Day of the Doctor (novelisation)"]) 

A number ofsilicon-based lifeforms were encountered by the Doctor, including theKastrians, (TV:The Hand of Fear)Ogri (TV:The Stones of Blood) andPlutonians. (PROSE:GodEngine)

Amessenger told theFourth Doctor that theTime Lords had foreseen a time where theDaleks had destroyed all other lifeforms; it was for this reason that theFourth Doctor was assigned themission of interfering with the Daleks'development. When the Doctor confrontedDavros about theevil 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 thisbelief, using it as justification and rationalisation for their crusade against theThals. (AUDIO:The Lights of Skaro,Return to Skaro)

In the29th century,Taren Capel tried to start arobot revolution, as he believedrobots to be a superior lifeform and wanted to liberate them. (TV:The Robots of Death)

CAIN was an artificial lifeform on theMyriad, with a fungoidbrain. (AUDIO:The Pyralis Effect)

Strangeicarons had no harmful effects on organic lifeforms, but interfered withstarshipjump engines. (PROSE:GodEngine) Conversely, normal icarons "preferred" to be inhyperspace and causedparanoia andpsychotic behaviour incarbon-based lifeforms. (PROSE:Original Sin)

Gabrielideans were aliquid-based lifeform. One of theirsoldiers fighting for theTime Lords intheir War againstthe enemy was captured and turned into an idea-based lifeform known as theShift. (PROSE:Alien Bodies)

Historians who had been allowed access to the "Monster Vaults" of thedatabanks inthe Doctor's TARDIS observed that everypart of anAbzorbaloff'sbody "fromhead totoe" was in a constant state ofsuction, thusconsuming any living thing upon contact. (PROSE:The Monster Vault,TV:Love & Monsters)

When theTenth Doctor andDonna Noble visitedthe Library, itscomputers confirmed they were the onlyhumanoid life present, but then told them there were over "amillion million lifeforms" in the Library. (TV:Silence in the Library)