A planetary class describing marginally habitable worlds, usually with a thin or argon-rich atmosphere and little animal life, where humanoids can survive but seldom comfortably, as several stranded Starfleet crews discover.
Class L planets sit just below Class M in the Federation's scale of habitability. They typically have a breathable but thin or unusual atmosphere, sparse vegetation and little or no native animal life, so that humanoids can survive on the surface but colonisation is difficult.
Several episodes turn on that marginal quality. In The Next Generation's The Chase, Indri VIII is a Class L world whose biosphere is destroyed by Klingons. In Deep Space Nine's The Ascent, Odo and Quark crash on a freezing Class L planet and must climb a mountain to call for help, and in The Sound of Her Voice Captain Lisa Cusak is marooned on one whose air must be countered with injections of tri-ox compound.
Voyager finds the descendants of abducted humans on a Class L world in The 37's, and the ship's alternate self is destroyed on a Class L ice planet in Timeless.