Webster's English Dictionary

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atom

at.om \'at-*m\ n [ME, fr. L atomus, fr. Gk atomos, fr. atomos indivisible, 
   fr.] a- + temnein to cut - more at TOME 1: one of the minute indivisible 
   particles of which according to ancient ma terialism the universe is 
   composed 2: a tiny particle : BIT  3a: the smallest particle of an element 
   that can exist either alone or in combination 3b: a group of such 
   particles constituting the smallest quantity of a radical 4: the atom 
   considered as a source of vast potential energy