Compomer is a polyacid-modified composite made by incorporating glass particles of GIC in water-free polyacid liquid monomer with appropriate initiator.  The rationale for using this material is the integration of the fluoride-releasing capability of glass ionomers with the durability of resin composites.  Compomers possess properties distinctly different from those of resin composites and glass ionomers.  Ref: Phillip’s Science of Dental Materials ed 12 pg 328