1. A substance that acts cohesively, see Cementitious Material. 2. Fines which hold gravel together when it is dry. 3. The nonvolatile portion of a paint which serves to bind or cement the pigment particles together; oils, varnishes and proteins are examples of binders; see vehicle. 4. A chemical additive to gypsum board core formulation, frequently starch, to improve the bond between the surfacing papers and the core.