Hybridisation
The mixing of atomic orbitals on the same atom to form an equal number of new, equivalent orbitals suited to bonding.
Unit
Dimensionless
In depth
Hybrid orbitals point towards the bonded atoms and overlap more effectively than the pure orbitals they came from, so the resulting bonds are stronger and equivalent. The number of hybrids equals the number of electron domains, which is why hybridisation and VSEPR geometry always agree.
Examples in the real world
Carbon's 2s and three 2p orbitals mix into four sp³ hybrids, explaining methane's four identical C–H bonds.