Resins are solid or semisolid complex amorphous mixtures of organic compounds that cannot be characterized by definite melting points and which show no tendency to crystallize. Historically, however, the term resin has been applied to a group of substances obtained as gums from trees or manufactured synthetically. But strictly speaking, resins are complex mixtures, whereas gums are compounds that can be represented by a single chemical formula.

The word gum was originally applied to any soft sticky product derived from trees; for example, the latex obtained from Hevea trees, which is the source of natural or gum rubber. Natural rubber, i.e, chemically unsaturated polyisoprene, is a polymeric material that can also be produced synthetically. Thus, although the term resin when applied to polymers actually antedates the understanding of the chemistry of polymers and originally referred to the resemblance of polymer liquids to the pitch on trees, it...