Lipids are a class of biomolecules, which have diverse structures and, therefore, can not be defined based on structural features.  Instead, lipids are defined based on a physical property, namely the solubility.  Lipids are biomolecules that are insoluble in water but soluble in nonpolar solvents, such as ether, chloroform, and dichloromethane.

see also saponifiable lipid, nonsaponifiable lipid