Quantum Number
Definition
One of four numbers that describe an electron's address: its energy level, orbital shape, orbital orientation, and spin.
Technical definition
The quantum numbers (n, l, ml, ms) are four numbers specifying an electron's state: n is the principal number (energy level), l is the azimuthal number (orbital shape), ml is the magnetic number (orbital orientation), and ms is the spin (+1/2 or -1/2). No two electrons in an atom share all four, which is the Pauli exclusion principle.
Example
An electron in a 2p orbital might have n=2, l=1, ml=0, ms=+1/2: a unique ID that no other electron in that atom can fully share.
Common confusion
Quantum numbers describe allowed states, not exact positions. They tell you which orbital an electron occupies, not where in the orbital it is right now.