If an oil or fat is hydrogenated, people have treated it with hydrogen so a liquid oil becomes firmer — more like a solid. Factories do this to turn cheap plant oils into something that spreads like butter, or that can sit on a shelf for a long time without going off. You see the word on food labels: hydrogenated oil, hydrogenated fat, partly hydrogenated vegetable oil. Chemists can hydrogenate other things too, but in ordinary English this word almost always points at food fats. Though actually, “partly hydrogenated” oils are the ones that make trans fats, which many countries now try to limit; fully hydrogenated fat is a different, harder product. The word sounds scientific and a bit industrial — not a compliment in food talk these days.