Hypervalency is the ability of an atom to form bonds beyond the lewis octet rule .. Hypervalent compounds are common for the second and subsequent row elements in groups 14–18 of the periodic table. A description of the hypervalent bonding implies a transfer of the electrons from the central (hypervalent) atom to the nonbonding molecular orbitals which it forms with (usually more electronegative)ligands.
So bascially hypervalent compounds have more than 8 electrons in their valence shell.