Tl rare dated informal slang noun 1. a soft gray malleable metallic element that resembles tin but discolors on exposure to air; it is highly toxic and is used in rodent and insect poisons; occurs in zinc blende and some iron ores Synonymsthalliumatomic number 81 Related words Broader terms metallic element Parts zinc blende WordNet 2.1 (Princeton University). WordNet 2.1 Copyright Princeton University. All rights reserved.
