Merriam-Webster Collegiate Dictionary - atom

 noun  Etymology: Middle English, from Latin ~us, from Greek ~os, from ~os indivisible, from a- + temnein to cut  Date: 15th century  1. one of the minute indivisible particles of which according to ancient materialism the universe is composed  2. a tiny particle ; bit  3. the smallest particle of an element that can exist either alone or in combination  4. the ~ considered as a source of vast potential energy