At

Definitions of At

1. noun

   a highly unstable radioactive element (the heaviest of the halogen series); a decay product of uranium and thorium
   synonyms: astatine, atomic number 85

   type of:

   chemical element, element

   any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter

   halogen

   any of five related nonmetallic elements (fluorine or chlorine or bromine or iodine or astatine) that are all monovalent and readily form negative ions