A silicon-based lifeform, also known as a silicon cycle life form or simply silicon creature, was a corporeal lifeform that used silicon as the basis of its structure and life functions rather than carbon.
While silicon-based lifeforms generally did not evolve on M-class planets; for a period of ten thousand years, Organian observers used a silicon-based virus on an M-class planet to see how other species treated their infected crews. (ENT: "Observer Effect")
In 2151, Captain Jonathan Archer of Enterprise NX-01 tried to convince Commander Charles Tucker III that Starfleet had sent them on a top secret classified mission to a planet to make first contact with a silicon-based lifeform, when Tucker was hallucinating "people coming out of the rock face." Archer told Tucker it was a major breach of security to reveal that information, because T'Pol was the only person granted clearance to speak with them, since they had supposedly made contact with the Vulcans before. (ENT: "Strange New World")
Silicon-based life was thought to be physiologically impossible before the discovery of the Horta of Janus VI in 2267. It was not possible for silicon life to evolve in an oxygen atmosphere. (TOS: "The Devil in the Dark") Later, when the Enterprise landing party was on the surface of the Kalandan outpost, James T. Kirk wondered aloud if it were possible that "the rocks have life", which reminded Sulu of the silicon creatures from Janus VI, to which McCoy reminded that unlike the rocks on the planet they were on, "our instruments recorded that. They were life forms. They registered as life forms." (TOS: "That Which Survives")