• A basic part of a whole. In chemistry, refers to a simple substance that cannot be broken down into smaller parts or changed into another substance. The basic part of an element is an atom, which contains protons, neutrons, and electrons. All atoms of an element have the same number of protons. Examples of elements are hydrogen, carbon, oxygen, nitrogen, and calcium.
  NCI Dictionary of Cancer Terms
  U.S. National Cancer Institute, 2021
• A substance whose atoms all have the same atomic number, that cannot be divided into simpler substances, and that singly or in combination constitute all matter.
  NCI
  U.S. National Cancer Institute, 2021
• Fundamental chemical units of which all matter at or above the atomic level is composed; each element is made up of atoms that are identical in number of electrons and protons and in nuclear charge, but may differ in mass or number of neutrons; such varieties of an element are isotopes.
  CRISP Thesaurus
  National Institutes of Health, 2006
• Substances that comprise all matter. Each element is made up of atoms that are identical in number of electrons and protons and in nuclear charge but may differ in mass or number of neutrons.
  NLM Medical Subject Headings
  U.S. National Library of Medicine, 2025