terbium - a metallic element of the rare earth group; used in lasers; occurs in apatite and monazite and xenotime and ytterbitemetal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc. apatite - a common complex mineral consisting of calcium fluoride phosphate or calcium chloride phosphate; a source of phosphorus gadolinite, ytterbite - a mineral that is a source of rare earths; consists of silicates of iron and beryllium and cerium and yttrium and erbium monazite - a reddish-brown mineral containing rare earth metals; an important source of thorium and cerium |