Stearyl-CoA desaturase that utilizes O(2) and electrons from reduced cytochrome b5 to introduce the first double bond into saturated fatty acyl-CoA substrates. Catalyzes the insertion of a cis double bond at the delta-9 position into fatty acyl-CoA substrates including palmitoyl-CoA and stearoyl-CoA (PubMed:11500518, PubMed:11533264, PubMed:16275639, PubMed:16443825, PubMed:26098370). Gives rise to a mixture of 16:1 and 18:1 unsaturated fatty acids (PubMed:11500518, PubMed:11533264, PubMed:16443825, PubMed:26098370). Plays an important role in lipid biosynthesis (PubMed:17127673, PubMed:10899171, PubMed:11500518, PubMed:11441127, PubMed:11533264, PubMed:12177411, PubMed:26098370). Plays an important role in regulating the expression of genes that are involved in lipogenesis and in regulating mitochondrial fatty acid oxidation (PubMed:12177411, PubMed:17127673, PubMed:24356954, PubMed:24295027). Plays an important role in body energy homeostasis (PubMed:17127673, PubMed:15210843, PubMed:24295027, PubMed:24356954). Contributes to the biosynthesis of membrane phospholipids, cholesterol esters and triglycerides (PubMed:10899171, PubMed:11500518, PubMed:11441127, PubMed:11533264, PubMed:12177411, PubMed:15210843, PubMed:26098370). Required for normal development of sebaceous glands (PubMed:17738154, PubMed:11533264). Required for the biosynthesis of normal levels of delta-9 unsaturated fatty acids and 1-alkyl-2,3-diacylglycerol in the Harderian gland (PubMed:11500518). Required for normal production of meibum, an oily material that prevents drying of the cornea (PubMed:11533264).