Syntactic foams are composite materials synthesized by filling a metal, polymer, cementitious or ceramic matrix with hollow spheres called microballoons or cenospheres or non-hollow spheres (e.g. perlite) as aggregates. In this context, "syntactic" means "put together." The presence of hollow particles results in lower density, higher specific strength (strength divided by density), lower coefficient of thermal expansion, and, in some cases, radar or sonar transparency.