Question

What are all atoms?

Answer 1

The fundamental structure of matter.

Explanation:

We now know that they are themselves composed of yet smaller particles, but those particles to not really exist (long-term) apart from their inclusion in an atom.

The atom gets its name from early Greek philosophers. The concept of a tiny “building block” of nature is a very old one espoused in ancient Greece and India. “Modern chemistry” beginning in the 1800’s experienced the first direct attempts to isolate atoms. See also:
http://www.livescience.com/37206-atom-definition.html
https://en.wikipedia.org/wiki/Atom
https://www.nyu.edu/pages/mathmol/textbook/atoms.html