1Chiefly Astronomy. A function involving relationships of mass; specifically (a) a relationship used to determine the masses of the two stars of a spectroscopic binary when only the spectrum of one star (m₁) can be observed, namely (m₂ 3 sin3 i)/(m₁ + m₂)2, where m₁ and m₂ are the star masses and i is the inclination of the orbit; (b) an expression describing the number of objects per unit volume in different ranges of mass, for a group of stars, galaxies, etc.

2Mathematics. In full "probability mass function". A function giving the probability that a discrete random variable will take a specific value.