the smallest quantity of some physical property, such as energy, that a system can possess according to the quantum theoryb. a particle with such a unit of energy2. amount or quantity, esp a specific amount3. (often used with a negative) the least possible amount that can suffice: there is not a quantum of evidence for your accusation.4. something that can be quantified or measured5. (modifier) loosely, sudden, spectacular, or vitally important: a quantum improvement.