Note: For Germanic verb and noun derivatives of *haita- see heat entry 1, heat entry 2. The Early Modern English shortening of Middle English long open o in hot has been explained as influence of the comparative and superlative forms, but this does not appear to have occurred in analogous cases. The Germanic adjective *haita-, from a presumed pre-Germanic *koid-, is reflected in other ablaut variants, as Gothic heito "fever," from *hītōn- (from *keid-) and a zero grade in Old Frisian hette, hitte "heat," Old High German hizzea, hizza (from *kid-); all these may reflect an unattested strong verb *hītan-. Traditionally the base *keid- has been connected by means of a "root extension" *-d- with Old High German hei, gehei, geheige "heat, drought," Middle Dutch hei "hot, dry," from a presumed Indo-European verb base *kei- "burn, heat" (in earlier literature *kai-, though there appears to be no reason to posit such a vocalism). These forms have in turn been compared with a series of Baltic words (as Lithuanian kaičiù, kaĩsti "to heat," kaistù, kaĩsti "to become hot"), from *koit- with a different root extension -t-.

Note: Frequently nominalized as a pair with cold entry 1 (the nominal equivalent of which is identical with the adjective), a connection that goes back to Old English ("hat and ceald").