An amide ( or or ), also known as an acid amide, is a compound with the functional group R_nE(O)_xNR'₂ (R and R' refer to H or organic groups). Most common are carboxamides (organic amides) (n = 1, E = C, x = 1), but many other important types of amides are known including phosphoramides (n = 2, E = P, x = 1 and many related formulas) and sulfonamides (E = S, x= 2). The term amide refers both to classes of compounds and to the functional group (R_nE(O)_xNR'₂) within those compounds.