Sebaceous is a doctor’s and biology-class word. It talks about the parts of the body that make oil for the skin. Your skin has tiny glands that squeeze out an oily stuff; those are sebaceous glands, and the oil itself is called sebum. That oil helps keep skin from drying out, though too much of it can make the face look shiny or help pimples form. You will almost never hear sebaceous in daily chat — people just say oily skin. It comes from Latin sebaceus, from sebum, meaning tallow or grease, plus an ending that means “of that kind.” A very old root of that Latin word is also the source of English soap, which fits in a rough way: both have to do with grease and the skin.