In classical physics there are 2 types of mass considered (and measured to be equal):
1. gravitational mass used in Newton's Law of Gravity
2. intertial mass used in Newton's Second Law
Measurements show that these two kinds of mass properties of an object are equal (are in linear proportion) to a very high precision. This was suspected and even Newton has considered and measured this, but the first really high precision measurements were done by baron Loránd Eötvös[1].
1. gravitational mass used in Newton's Law of Gravity
2. intertial mass used in Newton's Second Law
Measurements show that these two kinds of mass properties of an object are equal (are in linear proportion) to a very high precision. This was suspected and even Newton has considered and measured this, but the first really high precision measurements were done by baron Loránd Eötvös[1].
[1] https://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_experiment