An Etymological Dictionary of Astronomy and Astrophysics
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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.

From O.Fr. doublet, from → double + -et diminutive suffix.

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

The two → forbidden lines with → transitions from the upper → energy level  ¹D₂ of the doubly ionized oxygen atom. They occur in the green part of the → visible spectrum and are called → nebular lines. They are λ5006.8 ¹D₂-³P₂ and λ4958.9 ¹D₂-³P₁. Theoretical values for the [O III] 5007/4959 line-intensity ratio is 3.01 (Storey and Zeippen, 2000, MNRAS 312, 813).

[O III], spectroscopic notation of the ion O⁺⁺; → doublet.