pectin noun pec??tin \??pek-t??n\ (Medical Dictionary)Medical Definition of PECTIN 1: any of various water-soluble substances that bind adjacent cell walls in plant tissues and yield a gel which is the basis of fruit jellies 2: a commercial product rich in pectins and used chiefly in making jelly and other foods, in pharmaceutical products especially for the control of diarrhea, and in cosmetics L