What is the definition of "atom"

Atom is the substance from which molecule is made. and then solid substances are made.

An atom is a basic piece of matter. Eveything in the universe, except for energy is made of atoms. Atom is made of protons, neutron and electrons.

Name atom comes from Greek átomos, which means "indivisible", first person to use the name atom was Democritus, however we wasn't right about it being indivisible.