Fat is one of a class of substances called lipids. These include not only fats but also waxes (naturally found in plants); phospholipids, such as lecithin in egg yolk; and steroids, such as cholesterol, also found in egg yolk, and ergosterol, the chemical name of vitamin D.
Lipids belong to a larger class of chemical substances: esters. Esters are also important in food as flavouring compounds, some of which are oils and some are not. Esters which act as flavourings have an aromatic quality (in common language, not in the restricted chemical sense of ‘aromatic’). An example of ester fragrance is ethyl acetate (now called ethanoate), the chief component of the smell of strawberries. Esters with small, light molecules are fragrant because the molecules are light enough to drift about in a vapour. But the esters which make up the bulk of fats have rather large molecules, so that they form liquids or solids with no smell.