molecular orbital
An atomic orbital is the cloud-shaped region where an electron is likely to be found around a single atom. When atoms join into a molecule, those clouds merge and reshape themselves around the whole group of nuclei at once. The new, molecule-wide clouds are molecular orbitals — homes for electrons that belong to the molecule, not to any one atom.
Precisely, a molecular orbital is a wavefunction describing an electron that moves over two or more atomic nuclei in a molecule. Like atomic orbitals, each one can hold at most two electrons and has a definite energy. Electrons fill the available molecular orbitals from lowest energy upward, and the resulting pattern explains the molecule's stability, its magnetism, and even its colour.
Molecular orbital theory is one of the two great pictures of bonding (the other is valence bond theory). Its strength is that it treats electrons as belonging to the whole molecule, which neatly explains things simpler pictures miss — such as why oxygen gas is attracted to a magnet. Its cost is that the orbitals are spread out and less intuitive than tidy lines drawn between atoms.
In O₂, molecular orbital theory predicts that the last two electrons sit alone in two equal-energy orbitals rather than pairing up. That leaves two unpaired electrons, which is exactly why liquid oxygen is drawn to a magnet — a fact the simpler shared-pair picture cannot explain.
Oxygen's magnetism is the classic win for molecular orbital theory.
Do not confuse a molecular orbital with an atomic orbital. An atomic orbital surrounds one nucleus; a molecular orbital is built by combining atomic orbitals and spreads over several nuclei at once. The count is conserved: combining N atomic orbitals always produces exactly N molecular orbitals.