The Collaborative International Dictionary of English v.0.48:

lipid \lip"id\ n. [Gr. li`pos fat.] (Chem., Biochem.)
   Any of a variety of oily or greasy organic compounds found as
   major structural components of living cells; they are
   insoluble in water but soluble in organic solvents such as
   alcohol and ether, and include the common fats,
   cholesterol and other steroids, phospholipids,
   sphingolipids, waxes, and fatty acids; some of the
   lipids, together with proteins and carbohydrates, form an
   essential structural component of living cells, as in the
   cell walls and membranes. The term lipid refers to its
   solubility in nonpolar solvents, and has no significance with
   regard to chemical structure.

   Syn: lipide, lipoid.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

lipid
    n 1: an oily organic compound insoluble in water but soluble in
         organic solvents; essential structural component of living
         cells (along with proteins and carbohydrates) [syn:
         lipid, lipide, lipoid]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "lipid":
   animal oil, cerate, cholesterol, drying oil, essential oil, ester,
   fat, fatty oil, fixed oil, glyceride, grease, hydrogenated fat,
   lipin, lipoid, mineral oil, oil, oleum, polyunsaturated fat, sebum,
   steroid, tallow, unsaturated fat, vegetable oil, volatile oil