thal ‧ li ‧ um /ˈθæliəm/ BrE AmE noun [uncountable]
[ Date: 1800-1900 ; Language: Modern Latin ; Origin: Greek thallos 'new green plant part' ; because it has a bright green line in its spectrum ]
a soft blue-white metal that is poisonous, used in ↑ photoelectric cell s and to kill rats, mice, and insects. It is a chemical ↑ element : symbol Tl.