Mass

Mass is the what enables us to say that an object (or an idea) is weighty or massive.

Context

Issac Newton needed a mathematical definition of what caused gravity and so defined it thus<

The quantity of matter is the measure of the same, arising from its density and bulk conjucntly ... It It is this that I mean hereafter everywhere under the name body, or mass. And the same is known by the weight of each body, for it is proportional to the weight, as i have found by experiments on pendulums, very accurately made, which shall be shown hereafter.