As defined by Merriam-Webster - noun, concise Encyclopedia:
Any of a diverse group of organic compounds that are grouped together because they do not interact appreciably with water. One of the three large classes of substances in foods and living cells, lipids contain more than twice as much energy (calories) per unit of weight as the other two (proteins and carbohydrates). They include the fats and edible oils (e.g., butter, olive oil, corn oil), which are primarily triglycerides; phospholipids (e.g., lecithin), which are important in cell structure and metabolism; waxes of animal or plant origin; and sphingolipids, complex substances found in various tissues of the brain and nervous system. Since insolubility is the defining characteristic, cholesterol and related steroids, carotenoids (see carotene), prostaglandins, and various other compounds are also classifiable as lipids.
lipid definition. 2013. In Merriam-Webster.com. Retrieved Jan 27, 2013, from www.merriam-webster.com/dictionary/lipid