noun

• (chemistry, physics) The smallest possible amount of matter which still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.
  "Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste oxygen using solar energy. The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light. The critical component of the photosynthetic system is the water-oxidizing complex, made up of manganese atoms and a calcium atom."
• (history of science) A hypothetical particle posited by Greek philosophers as an ultimate and indivisible component of matter.
• (now generally regarded figuratively) The smallest, indivisible constituent part or unit of something.
  "Towards the following morning, the thermometer fell to 5°; and at daylight, there was not an atom of water to be seen in any direction."
• (philosophy) In logical atomism, a fundamental fact that cannot be further broken down.