Subatomic means smaller than an atom. A subatomic particle, the subatomic world, subatomic physics.
The atom was named from the Greek for uncuttable, so the word is a contradiction that history forced on us: the electron was found in 1897 and the nucleus in 1911. At that scale things stop behaving in ways any everyday picture can describe.
1adjectivesubatomicSmaller than an atom.
“Protons and electrons are subatomic particles.”
subatomic is a scale.
From sub- + atomic.
Etymology from Wiktionary, CC BY-SA