An acidic substance, later identified with homogentisic acid, which is excreted by people with alkaptonuria and causes their urine, left standing, to turn dark through oxidation.
Homogentisic acid is 2,5-dihydroxyphenylacetic acid; C₆H₄(OH)₂COOH.
Mid 19th century; earliest use found in Edmund Parkes (1819–1876), physician and hygienist. From German Alkapton, †Alcapton from al- (in Alkali) + ancient Greek κάπτον, neuter present participle of κάπτειν to swallow greedily; see quot. 1860 for the semantic motivation of the name.