An Etymological Dictionary of Astronomy and Astrophysics
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A forbidden line emitted by interstellar ionized gas by several atomic species (O, O⁺, O⁺⁺, N⁺, S⁺⁺, etc.) corresponding to the transition from the electronic state 1S to 3P. Examples are the ultraviolet line of the doubly ionized oxygen [O III] at 2321 A and [N II] 3063 A. → auroral line; → forbidden line; → nebular line.

→ trans-; → auroral line.