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).
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)
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)