Involved in the final reduction of the elongation cycle of fatty acid synthesis (FAS II). Catalyzes the NADH-dependent reduction of a carbon-carbon double bond in an enoyl moiety that is covalently linked to an acyl carrier protein (ACP). It can use both crotonyl-CoA and crotonyl-ACP. a 2,3-saturated acyl-[ACP] + NAD(+) = a (2E)-enoyl-[ACP] + H(+) + NADH a 2,3-saturated acyl-CoA + NAD(+) = a (2E)-enoyl-CoA + H(+) + NADH (2E)-butenoyl-[ACP] + H(+) + NADH = butanoyl-[ACP] + NAD(+) butanoyl-CoA + NAD(+) = (2E)-butenoyl-CoA + H(+) + NADH Weakly inhibited by triclosan. Lipid metabolism; fatty acid biosynthesis. Monomer. Belongs to the TER reductase family.