'Thermal' is Greek for 'heat' — from PIE *gwher- (warm). The same root gave English 'warm' via Germanic.
Of, relating to, or caused by heat; designed to retain body heat.
From Greek 'thermē' (heat, warmth) via Modern Latin 'thermalis,' with the suffix -al added in scientific usage from the 18th century onward. Greek 'thermē' derives from 'thermos' (hot, warm), from PIE *gʷʰer- (warm, hot), a root that also produced Latin 'formus' (warm), Sanskrit 'gharmá-' (heat, warmth), and Armenian 'jerm' (warm). The English word entered scientific and geological vocabulary to describe warm springs ('thermal spring'), then expanded to atmospheric columns of rising warm air in aviation, and later to heat-