a bright, silvery chemical element, one of the rare-earth elements: symbol, Tm; at. no., 69
See thulium in American Heritage Dictionary 4
(tho͞oˈlē-əm, thyo͞oˈ-)
noun Symbol Tm
A bright, silvery rare-earth element obtained commercially from monazite, having an x-ray emitting isotope that is used in small portable medical x-ray units. Atomic number 69; atomic weight 168.934; melting point 1,545°C; boiling point 1,727°C; specific gravity 9.3; valence 2, 3. See Table at element.