In chemistry, an ester is a compound derived from an acid (organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group (−OH) of that acid is replaced by an organyl group (−R). Analogues derived from oxygen replaced by other chalcogens belong to the ester category as well (i.e. esters of acidic −SH, −SeH, −TeH, −PoH and −LvH groups). According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well (e.g. amides), but not according to the IUPAC.