chiefly British English spelling of ardor (q.v.); for spelling, see -or.

"heat of passion or desire," mid-15c., ardour, from Old French ardure "heat, glow; inflammation; passion" (12c., Modern French ardeur), from Latin ardorem (nominative ardor) "a flame, fire, burning, heat;" also of feelings, etc., "eagerness, zeal," from ardere "to burn" (from PIE root *as- "to burn, glow"). In Middle English used of base passions; since Milton's time of noble ones.