An atom is defined as the basic unit of a chemical element. A fundamental piece of matter. Plain simple and basic chemistry concept.
Obviously we now know that atoms are made of smaller particles, called subatomic particles: protons (positively charged by convention), electrons (negatively charged by convention) and neutrons (neutral particles).
In simple terms: Protons and neutrons make up the inner part of the atom, or nucleus, and account for most of the mass of the atoms. Electrons make up a cloud (orbitals, or areas in which the probability of finding an electron is high) around the nucleus.
Protons and neutrons are not fundamental particles; they are made of quarks. On the other hands, as far as we known, electrons are fundamental particles and they are not made of anything smaller.