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