The term mass ratio, i.e., the ratio of two masses (or of a mass with some other quantity) has many uses. One astrophysics use is the ratio of masses of two bodies coorbiting, such as a binary star or a planet and its host star. Often the mass ratio is a quantity that can be determined: it matches their radial velocity ratio which can be determined through their Doppler shift for a double-line spectroscopic binary. If additional information can be determined, e.g., the absolute velocities (if the orbital inclination can be determined), or the total mass, then the individual masses can be determined.
For GW detections of mergers of two compact objects, each either a black hole or neutron star, the mass ratio is one of the quantities that can be determined, but only with limited precision since it is revealed by the fine detail of the chirp (by higher order term(s) in an equation expressed as a series), thus the imprecision of the detector is effectively amplified, for this purpose.
The symbol μ is sometimes used for the mass ratio of binary stars and the symbol q for GW detections.
Astrophysics also uses the term mass ratio in relation to relative abundances of hydrogen, helium, and metals compared to that of all baryonic matter.
The term mass ratio is common in other fields such as chemistry. In in rocketry and aeronautics, it is commonly used to compare the mass of an engine (or entire craft) to the thrust it produces.