1. A core, or nucleus, of protons and neutrons that are surrounded by an orbiting cloud of electrons: The chemical behavior of an atom is largely determined by the distribution of its electrons, particularly the number that exist in its outermost level.

Carbon, the sixth element of the periodic table of atoms, forms the basis of most biological molecules and oxygen is another atom, one of about a hundred basic elements that make up all of the objects around us.

2. The fundament building blocks for all matter: An atom is the smallest representative sample of a chemical element and it consists of a positively charged nucleus surrounded by negatively charged electrons.

All of the matter on the Earth is made of atoms which are the chemical structures of our world.

New modern atomic theory is generally credited to an English meteorologist, John Dalton (1766-1844), who suggested that for each chemical element there was a corresponding species of indivisible objects called atoms.

Two or more atoms that are joined together form a molecule which is the same term that applies to any cluster of atoms which can be isolated, whether it contains two atoms or a thousand atoms.

3. A tiny particle or a very small amount of anything: After the politician's presentation, Nicola was heard to say that there was not an atom of truth in anything that he had said.
4. Etymology: atom means "not cut, indivisible"; from Greek atomos, "uncut, indivisible"; from a-, "not" + tomos, "a cutting"; from temnein, "to cut".

A system in which the nuclei of two colliding atoms briefly overlap: So, such quasi atoms seem to be one atom with a combined atomic number.