In fatty acid biosynthesis chain termination and release of the free fatty acid product is achieved by hydrolysis of the thio ester by a thioesterase I, a component of the fatty acid synthetase complex. The chain length of the released fatty acid is usually C16. However, in the mammary glands of non-ruminant mammals, and in the uropygial gland of certain waterfowl there exists a second thioesterase which releases medium-chain length fatty acids (C8 to C2). (9Z)-octadecenoyl-[ACP] + H2O = (9Z)-octadecenoate + H(+) + holo-[ACP] Belongs to the thioesterase family.