What is a “ medium” in regards to waves ?

Match the vocabulary terms to their definitions. 1 . a long, chain-like set of molecules made up of repeating units joined end to end polymer 2 . a hard, brittle, heat- and corrosion-resistant material made by subjecting a nonmetallic mineral mixture to intense heat ceramic 3 . a plastic with low elongations that cannot be recycled thermoset 4 . a carbon fiber embedded in a polymer resin matrix thermoplastic 5 . a plastic with high elongations that can be recycled crystal 6 . a solid form resulting from the arrangement of atoms, ions, or molecules in definite geometric patterns composite