(1) The smallest unit of matter indivisible by chemical means.
(2) The fundamental building block of a chemical element.
(3) The smallest possible unit of an element that still has all the chemical properties of that element.
An atom is consisting of a nucleus surrounded by one or more shells of electrons.
Word origin: from the Greek átomos, which means uncuttable, something that cannot be divided further.
Related forms: atomic (adjective).