means Relating to heat — or, as a noun, a rising column of warm air that lifts birds and gliders.

from From Greek 'thermos,' meaning hot or warm, the same root that warms 'thermometer,' 'thermos,' and 'thermodynamics.' It entered English by the early 19th century as the language of science reached for a precise word for heat. The aviator's sense — the buoyant updraft of sun-warmed air — is a younger relative, born once people started riding those invisible currents.