density: the mass per unit volume of an object
gravitational field: a field created by any object with mass, extending outward in all directions, which determines the influence of that object on all others
gravity: a force of attraction towards the centre of objects
inverse square law: any physical law stating that some quantity is inversely proportional to the square of the distance from a point
mass: the property of physical objects that, roughly speaking, measures the amount of matter they contain
Newton's law of universal gravitation: summarized mathematically as
proportionality constant: the constant value of the ratio of two proportional quantities
torsion balance: a measuring instrument designed to measure small forces by the torsion (twisting) they exert on a thin wire
universal gravitational constant: the value G in the equation summarizing Newton's law of universal gravitation