Two stationary states which have the same orbital and spin angular momentum but which have different total angular momenta, and therefore have slightly different energies due to spin-orbit coupling.

See <<dipole>>

A source and a sink separated by an infinitesimal distance, each having an infinitely large strength so that the product of this strength and the separation is finite.

A lens made up of two components, especially an achromat.

Two elementary particles which have slightly differing masses and the same baryon number, spin, parity, and charge conjugation parity (if self-conjugate), but have different charges.

Two electrons which are shared between two atoms and give rise to a nonpolar valence bond.

Two closely separated spectral lines arising from a transition between a single state and a pair of states forming a doublet as described in the atomic physics definition.