Forbidden lines refer to optical spectral lines in astrophysics that are not typically observed in thermal equilibrium gas. These lines are called "forbidden" because they arise from transitions between energy levels that are unlikely to occur in a gas at thermal equilibrium. Forbidden lines are important in the study of highly ionized plasmas, such as those found in planetary atmospheres, supernova remnants, and active galactic nuclei. The study of forbidden lines can provide valuable information about the physical conditions and processes occurring in these environments.