Refers to a chemical that easily goes into solution in lipids. This is generally because it is nonpolar. Things that are fat-soluble are not very water-soluble, and vice versa.

Fat-soluble vitamins, notably vitamin E and vitamin A are an important part of nutrition, because they can preserve fatty tissues and stabilize dietary fat. However, they also present some danger of toxicity, whereas it's almost impossible to hurt yourself with a water-soluble vitamin like vitamin C. This is because the body's elimination mechanisms mostly involve water (urination, sweating, vomiting). Every time you pee, you get rid of any excess water-soluble chemicals, so it's hard for them to build up. But vitamin A, for example, can go into your body fat and stay there, only very slowly leaching out into the blood and eventually into the toilet. Thus, if you take too much each day, you can eventually become ill.

My skin once turned orange as a result of eating seriously immense quantities of carrots over many months. This condition, hypercarotenemia, took over a year to resolve once I quit.

However, fat-soluble vitamins should not be eschewed because of this danger. Their health benefits far outweigh the risks, when you consider that huge doses, even by megavitamin supplement standards, must be consumed to create a problem.