Quantum Number
Definition
The set of numbers that completely describes an electron's state in an atom — its address in the quantum world.
Technical definition
Four quantum numbers specify an atomic electron: principal n (energy level/shell), azimuthal l (orbital shape), magnetic m_l (orientation), spin m_s (±½). The Pauli exclusion principle forbids two electrons sharing all four.
Example
Hydrogen's ground-state electron is (n=1, l=0, m_l=0, m_s=+½) — that quartet is its complete quantum address.
Common confusion
Quantum numbers are not just labels — they correspond to quantized physical observables (energy, angular momentum). And ‘spin’ is not literal spinning; it is intrinsic angular momentum with no classical analogue.