in chemistry, word-forming element used to form the names of salts from acids in -ic; from Latin -atus, -atum, suffix used in forming adjectives and thence nouns; identical with -ate (1).
The substance formed, for example, by the action of acetic acid (vinegar) on lead was described in the 18th century as plumbum acetatum, i.e. acetated lead. Acetatum was then taken as a noun meaning "the acetated (product)," i.e. acetate. [W.E. Flood, "The Origins of Chemical Names," London, 1963]
before vowels chem-, word-forming element denoting "relation to chemical action or chemicals," from combining form of chemical (adj.), used to form scientific compound words from c. 1900. In 19c., chemico- was used.