resin1a : any of various solid or semisolid amorphous fusible flammable natural organic substances that are usually transparent or translucent and yellowish to brown, are formed especially in plant secretions, are soluble in organic solvents (as ether) but not in water, are electrical nonconductors, and are used chiefly in varnishes, printing inks, plastics, and sizes and in medicine
2a : any of a large class of synthetic products that have some of the physical properties of natural resins but are different chemically and are used chiefly in plastics b : any of various products made from a natural resin or a natural polymer (Merriam-Webster Online)