1. Any of a heterogeneous group of fats and fatlike substances characterized by being water-insoluble and being extractable by nonpolar (or fat) solvents such as alcohol, ether, chloroform, benzene, etc.
2. “Fat-soluble”, an operational term used to describe a solubility characteristic, not a chemical substance, i.e., denoting substances extracted from animal or vegetable cells by nonpolar or “fat” solvents; included in the heterogeneous collection of materials thus extractable are fatty acids, glycerides and glyceryl ethers, phospholipids, sphingolipids, alcohols and waxes, terpenes, steroids, and “fat-soluble” vitamins A, D, and E.