Carbon-based lifeform
A carbon-based lifeform was any lifeform, including flora and fauna, that depends upon carbon atoms, and was almost exclusively found on M class planets. (; ) 

It was believed to be scientifically impossible for carbon cycle lifeforms to inhabit a planet like Excalbia, which had a poisonous atmosphere and surface consisting mostly molten lava, nonetheless, the planet was inhabited by a technologically advanced species. ()

Humans, for example, were carbon-based. The Starfleet Academy course "Exobiology 101" taught several basics on carbon-based life, such as that the Human immune system was unable to combat non-carbon elements, such as silicon and silicon-based viruses, which could attack the Human body in unpredictable ways. () 

V'ger and its Ilia probe used the term carbon unit to describe the humanoid crew of the . V'ger considered carbon units to be inferior, and not true lifeforms. ()

The internal environment of Gomtuu was suitable for carbon-based lifeforms. ()

In spite of not being carbon-based, the silicate-based lifeform discovered on L-S VI in 2370 was determined to be vegetative, very much alive, and possibly akin to Odo. ()

While scanning Suspiria's array for sporocystian lifeforms, the  discovered instead the signatures of over two thousand carbon-based lifeforms. ()

A species of naturally-occurring photonic lifeforms encountered by Voyager in 2375 were unaware of the existence of biochemical lifeforms. They believed that photonics were living, and that carbon-based lifeforms were artificial. ()

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Lifeform classifications