Quantum
Definition
The smallest indivisible chunk of something — like energy or light — that nature allows. At the smallest scales, energy is not a smooth flow; it comes in discrete packets called quanta.
Technical definition
A discrete, indivisible unit of a physical quantity. In quantum mechanics, quantities such as energy, angular momentum, and charge can only take values that are whole-number multiples of a fundamental quantum.
Example
An electron in a hydrogen atom can only occupy specific energy levels — it cannot sit halfway between them. When it drops from a higher level to a lower one, it emits a single quantum of light (a photon) carrying exactly the energy difference.
'Quantum' does not mean 'very small' in general, and a 'quantum leap' in everyday speech means a huge jump — but in physics a quantum leap is the smallest change an electron can make. Also, 'quantum' is singular; 'quanta' is the plural.