An Etymological Dictionary of Astronomy and Astrophysics

A pair of forbidden spectral lines of the same atom arising from a common upper excitation level, for example [O III] λ 5007 and [O III] λ 4959, [N II] λ 6584 and [N II] λ 6548, and [S II] λ 6717,
[S II] λ 6731.
A pair of associated lines arising from transitions having a common lower energy level in a spectrum characteristic of the alkali metals.

Etymology (EN): From O.Fr. doublet, from → double + -et diminutive suffix.

Etymology (PE): Dotâyé, from dotâ→ double + -(y)é nuance suffix.