A maser source in which the methanol molecule (CH₃OH) is excited to maser action. Methanol masers are found only in star-forming regions, where they are second only to H₂O (water) masers in brightness. There are two classes of methanol masers: class I, excited by collisions, and class II, excited by infrared radiation. The most important class I masers are at a frequency of 44.1 GHz, while the most important class II masers, found only in regions where high-mass stars are forming, are at a frequency of 6.7 GHz.