Pronunciation: (kol'oid),[key] —n. 1. Physical Chem.a substance made up of a system of particles with linear dimensions in the range of about 10-7 to 5 × 10-5 cm dispersed in a continuous gaseous, liquid, or solid medium whose properties depend on the large specific surface area. The particles can be large molecules like proteins, or solid, liquid, or gaseous aggregates and they remain dispersed indefinitely. Cf. aerosol, emulsion, gel, sol4, suspension. 2. Med.a colloidal substance in the body, as a stored secretion or a cyst.