Medical Definition of resin
1a: 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, and are electrical nonconductorsb: rosinc: a solid pharmaceutical preparation consisting chiefly of the resinous principles of a drug or drugs usually extracted by solvents <resin of jalap>
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 plasticsb: any of various products made from a natural resin or a natural polymer
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