Also called non-volatile or carrier oil, a fixed oil is derived by pressing the seeds of a plant.* It won’t evaporate at room temperature and will leave a stain. It is insoluble in water.
Fixed oils are used in making soaps and lotions, and also as solvents for extracting plant properties. Glycerin is a by-product of fixed oils.
*Fixed oils are also derived from animals, but this site isn’t about that. 🙂
Related: essential oil, glycerin