[IBM]

What is meant by 'molecular orbital'? Could you explain with an example?

[Corribus]

A molecular orbital is a visual representation of the (real component of the) electron probability distribution (wavefunction) in space. Essentially, it is a visual representation of where electrons of a particular energy/momentum are most likely to be. Chemists use orbitals to understand bonding and chemical reactivity, which are mostly determined by interactions between electrons surrounding neighboring atoms/molecules.

It's important to understand that, because the mathematical equations governing the wavefunctions of physical systems having more than one electron are not possible to solve exactly, molecular orbitals are "only" (very good) approximations of the real properties of electrons. Nevertheless, molecular orbital theory is one of the most successful theories in modern chemistry, able to predict the properties of molecules accurately in an astounding number of occasions. But, it does have its failings, and not all bonding, reactivity, and chemical structure theories are based on molecular orbitals.