Definition of thallium
: a soft poisonous metallic element that resembles lead in physical properties, occurs sparsely in a number of common ores, and is used chiefly in the form of compounds especially in photosensitive devices and formerly as a pesticide — see element table
Origin of thallium
New Latin, from Greek thallos green shoot; from the bright green line in its spectrum
First Known Use: 1861
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