Hi! If you mean the definition of Atom, Atom is the smallest particle. It made up every matter we can see, feel, smell. etc. like solid, plasma, gas, liquid. Inside it, consist of proton which is positively charge, neutron which has no charge or neutral and electron which has negative charge. The neutron and proton build up the nucleus which is orbited by the electrons in shells or orbital path. The nucleus build up most of the atoms weight. Oh, by the way did I tell you that neutron, electron and proton have there own parts called quarks?
Alright, that's it for now, BYE!