The lipid class is defined by their solubility in nonpolar solvents. If biological material is shaken with a mixture of a nonpolar solvent and polar solvent, the fraction dissolving in the nonpolar solvent is the lipid fraction. Lipid molecules have these properties because they consist largely of long hydrocarbon tails which are hydrophobic in nature.

Because they are categorized based on solubility properties, rather than strict structural criteria, the category of lipids is a looser one than proteins, carbohydrates, or nucleic acids. The category of lipids encompasses not only the fatty acid/glyceride based forms of triglyceride and phospholipid but also diverse forms such as terpenes, prostaglandins and steroids.