(1) (Atom) For the Intel chip, see
Intel Atom.
(2) (Atom) For the Web feed, see
Atom syndication format.
(3) A single element. See
atomic.
(4) A fundamental building block of matter. Atoms are microscopic solar systems made up of particles and mostly space. Within that space, electrons create an outer shell by circling a nucleus containing protons and neutrons of similar mass.
Neutrons have no electrical charge, but protons have a positive charge. Because there are the same number of electrons as there are protons, and because electrons have a negative charge, the atom has a net charge of zero. See
electron.
Since the days of ancient Greece, the atom was considered "the" smallest element of matter and indivisible, which is what "atomic" means. However, in the 1960s, we learned that the nucleus is made up of more particles, named quarks and leptons. Perhaps some day, we will find even more particles in our never ending quest to discover what we are really made of. For an enlightening description of matter and the atom, see
Does it matter?.