Any natural or synthetic organic compound consisting of a non-crystalline or viscous liquid substance. Natural resins are organic substances that are transparent or translucent, formed in plant secretions. Synthetic resins comprise a large class of synthetic products that have some of the physical properties of natural resins but are different chemically. Most synthetic resins are polymers. The term resin dates from the early years of the plastics industry; it originally referred to naturally occurring amorphous solids such as shellac and rosin. See also plastics.