Molecular Orbital Theory: A theory that describes the molecular structure by considering electrons to be delocalized over an entire molecule forming molecular orbitals.
$\sigma$ Bond: A type of covalent bond formed by the head-on overlap of atomic orbitals, resulting in a symmetrical electron distribution along the axis connecting two nuclei.
$\pi$ Bond: A type of covalent bond formed by the side-to-side overlap of atomic orbitals, with electron density above and below the plane of the nuclei.