noun

• (now chiefly South Asia or law) The total amount of something; quantity.
  "The reader will perhaps be curious to know the quantum of this present, but we cannot satisfy his curiosity."
• (now chiefly South Asia or law) The total amount of something; quantity.
• (now chiefly South Asia or law) The total amount of something; quantity.
• (physics) The smallest possible, and therefore indivisible, unit of a given quantity or quantifiable phenomenon.
  "The quantum of light energy was later called a photon."

adj

• (informal) Of a change, significant.
• (physics) Involving quanta, quantum mechanics or other aspects of quantum physics.
  "Some of the most beautiful and thus appealing physical theories, including quantum electrodynamics and quantum gravity, have been dogged for decades by infinities that erupt when theorists try to prod their calculations into new domains. Getting rid of these nagging infinities has probably occupied far more effort than was spent in originating the theories."
• (physics) Involving quanta, quantum mechanics or other aspects of quantum physics.