An atom (in the context of physics and chemistry) is the smallest piece of matter that can exist of a particular type of substance.
Atoms can be subdivided into smaller particles, but then it ceases to be that substance.
The diameter of an atom (considered approximately spherical) is between about $1$ and $5$ angstroms.
The classical model of the atom is as a hard spherical body which is electrically neutral.
In this model, atoms exert no forces on one another until they are brought into contact, at which point they cannot be brought any closer together, no matter how hard they are pushed to do so.
The nucleus of an atom is the relatively dense core at its center.
It consists (to a first degree of approximation) of a number of protons and a similar number of neutrons.
The word atom comes from the Greek ἄτομον, meaning unbreakable or indecomposable.
It is pronounced with a short a, as at-tom, as opposed to ay-tom.