All fats are molecules composed primarily of carbon and hydrogen atoms.
A saturated fat has the maximum possible number of hydrogen atoms attached to every carbon atom. When a fat is missing pairs of hydrogen atoms it is called an unsaturated fat. Monounsaturated fat is missing one pair. Fats that are missing more than one pair of hydrogen atoms are labeled polyunsaturated. The greater degree of unsaturation in a fat, the lower the melting point of the fat.