what is hybridisation?

Hybridisation is the theoretical concept of mixing of atomic orbitals into new hybrid orbitals with different energies, shapes, etc., than the component atomic orbitals suitable for the pairing of electrons to form chemical bonds in valence bond theory.It was introduced to explain shapes of molecules or equivalency of bonds etc.Depending on what types of orbitals undergoing hybridisation,it may be sp,sp²,sp³,sp³d,sp³d² etc.For ex- in methane there is sp3 hybridisation.
Conditions for hybridisation--
1.Orbitals taking part in hybridisation must have small differences of energies.
2.The hybridised orbitals have equivalent energies and identical shapes.
3.The number of hybrid orbitals is equal to the number of orbitals taking part in hybridisation.
4.Both haif-filled as well as completely filled orbitals of valence shell can take part in hybridisation.
5.The hybrid orbitals can form stronger bonds as compared to the unhybridised or pure atomic orbitals because they can undergo more effective overlap.