| Noun | 1. | atomic number 40 - a lustrous grey strong metallic element resembling titanium; it is used in nuclear reactors as a neutron absorber; it occurs in baddeleyite but is obtained chiefly from zirconmetal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc. baddeleyite - a mineral consisting of zirconium oxide zircon, zirconium silicate - a common mineral occurring in small crystals; chief source of zirconium; used as a refractory when opaque and as a gem when transparent |