For binary stars, the mass function (or binary mass function) is a formula of observable values that yields constraints on the masses of the two stars, usable if their radial velocities can be determined through Doppler shifts.
P ——— V³ 2πG
This value is equal to the following constraint on the masses:
m³ —————— sin³i (M+m)²
The inclination is generally presumed negligible if it also transits, and may be roughly approximated by an "average" inclination if not. For star population studies, useful statistics may be derived assuming the inclinations are random.
Note that the phrase, mass function is to indicate it deals with physical masses. The phrase, mass function is also used in probability, for an incompatible concept: a probability mass function, which is a function yielding the probability that some discrete random variable takes on a given specific value.