The Collaborative International Dictionary of English v.0.48:

Fat \Fat\, n.
   1. (Physiol. Chem.) An oily liquid or greasy substance making
      up the main bulk of the adipose tissue of animals, and
      widely distributed in the seeds of plants. See Adipose
      tissue, under Adipose.
      [1913 Webster]

   Note: Animal fats are composed mainly of three distinct fats,
         tristearin, tripalmitin, and triolein, mixed in varying
         proportions. As olein is liquid at ordinary
         temperatures, while the other two fats are solid, it
         follows that the consistency or hardness of fats
         depends upon the relative proportion of the three
         individual fats. During the life of an animal, the fat
         is mainly in a liquid state in the fat cells, owing to
         the solubility of the two solid fats in the more liquid
         olein at the body temperature. Chemically, fats are
         composed of fatty acid, as stearic, palmitic, oleic,
         etc., united with glyceryl. In butter fat, olein and
         palmitin predominate, mixed with another fat
         characteristic of butter, butyrin. In the vegetable
         kingdom many other fats or glycerides are to be found,
         as myristin from nutmegs, a glyceride of lauric acid in
         the fat of the bay tree, etc.
         [1913 Webster]

   2. The best or richest productions; the best part; as, to
      live on the fat of the land.
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   3. (Typog.) Work. containing much blank, or its equivalent,
      and, therefore, profitable to the compositor.
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   Fat acid. (Chem.) See Sebacic acid, under Sebacic.

   Fat series, Fatty series (Chem.), the series of the
      paraffine hydrocarbons and their derivatives; the marsh
      gas or methane series.

   Natural fats (Chem.), the group of oily substances of
      natural occurrence, as butter, lard, tallow, etc., as
      distinguished from certain fatlike substance of artificial
      production, as paraffin. Most natural fats are essentially
      mixtures of triglycerides of fatty acids.
      [1913 Webster]