A forbidden line (or nebular line) is a spectral line appearing in astronomical bodies that is not reproduced in labs because they cannot sufficiently rarefy a sufficient amount of gas to produce an observable line. Mechanisms producing them are termed forbidden mechanisms. The lines form from electron orbit transitions that are sufficiently improbable that their timescale is much longer than that of collisions between atoms that trigger more substantial transitions, making them vary rare, and the phrase "highly improbable line" is perhaps more descriptive than forbidden line. They occur when an electron's state of excitation is metastable, i.e., energy is required to "get over a hump" to make the move to a lower-energy state that would produce the line, and outside influences such as collisions are likely to trigger other transitions. In space, a gas cloud can be thin enough to reduce collisions, yet large enough that these improbable forbidden transitions are numerous enough for our detection. They are seen in the thin upper Earth atmosphere as well as HII regions and planetary nebulae. Examples:
The square-bracket form [N II] indicates forbidden transitions by the given species (e.g., "N II" being nitrogen with a state of ionization of 1, i.e., singly ionized).