禁线或禁止机制（forbidden mechanism, forbidden line）是化学上的概念，它是原子在量子力学通常的选择规则下不被接受的能量转移发射谱线。在化学，「被禁止的」意义是在理想的对称情况下，自然的法则下绝对不可能的。虽然这种转换是在「技术上被禁止的」，但它们自然发生的机率并不是零。如果原子或分子被激发至受激状态，虽然蜕变概率是极端的低，但是原子或分子仍然可能做一个允许的跃迁，经由其它另行激发状态，进入较低的能阶，而它几乎一定会这样做。

禁线的发射谱线只在极低密度的外太空或地球极端的高层大气的气体或电浆中出现。即使在地球上的实验室中所能制造的最高等级的真空，密度依然太高，以至于禁线在仍未产生前，原子或分子就已经被碰撞而被再激发。但是，在太空的环境中，密度是每立方公分只有几个原子，使得原子间的碰撞不易发生。在这样的情况下，禁线的转移就有可能占发射光子中不能忽视的百分比。

In spectroscopy, a forbidden mechanism (forbidden transition or forbidden line) is a spectral line associated with absorption or emission of light by atomic nuclei, atoms, or molecules undergoes a transition that is not allowed by a particular selection rule but is allowed if the approximation associated with that rule is not made. For example, in a situation where, according to usual approximations (such as the electric-dipole approximation for the interaction with light), the process cannot happen, but at a higher level of approximation (e.g. magnetic dipole, or, electric quadrupole) the process is allowed but at a much lower rate.