An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar atmospheres, or various other conditions in interstellar space. Background Discrete transition energy Like a laser, the emission from a maser is stimulated (or seeded) and monochromatic, having the frequency corresponding to the energy difference between two quantum-mechanical energy levels of the species in the gain medium which have been pumped into a non-thermal population distribution. However, naturally occurring masers lack the resonant cavity engineered for terrestrial laboratory masers. The emission from an astrophysical maser is due to a single pass through the gain medium and therefore generally lacks the spatial coherence and mode purity expected from a laboratory maser. Nomenclature Due to the differences between engineered and naturally occurring masers, it is often stated that astrophysical masers are not "true" masers because they lack oscillation cavities. However, the distinction between oscillator-based lasers and single-pass lasers was intentionally disregarded by the laser community in the early years of the technology. This fundamental incongruency in language has resulted in the use of other paradoxical definitions in the field. For example, if the gain medium of a (misaligned) laser is emission-seeded but non-oscillating radiation, it is said to emit amplified spontaneous emission or ASE. This ASE is regarded as unwanted or parasitic (some researchers would add to this definition the presence of insufficient feedback or unmet lasing threshold): that is, the users wish the system to behave as a laser. The emission from astrophysical masers is, in fact, ASE but is sometimes termed superradiant emission to differentiate it from the laboratory phenomenon. This simply adds to the confusion, since both sources are superradiant. In some laboratory lasers, such as a single pass through a regeneratively amplified Ti:Sapph stage, the physics is directly analogous to an amplified ray in an astrophysical maser. Furthermore, the practical limits of the use of the m to stand for microwave in maser are variously employed. For example, when lasers were initially developed in the visible portion of the spectrum, they were called optical masers. Charles Townes advocated that the m stand for molecule, since energy states of molecules generally provide the masing transition. Along these lines, some use the term laser to describe any system that exploits an electronic transition and the term maser to