What is the difference between mass and weight?
Weight
In modern scientific usage, weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, whereas weight is a "force" that results from the action of gravity on matter: it measures how strongly the force of gravity pulls on that matter. However, in most practical everyday situations the word "weight" is used when, strictly, "mass" is meant. For example, most people would say that an object "weighs one kilogram", even though the kilogram is a unit of mass.
Quality (philosophy)
John Locke presented a distinction between primary and secondary qualities in "An Essay Concerning Human Understanding". For Locke, a quality is an idea of a sensation or a perception. Locke further asserts that qualities can be divided in two kinds: primary and secondary qualities. Primary qualities are intrinsic to an object—a thing or a person—whereas secondary qualities are dependent on the interpretation of the subjective mode and the context of appearance. For example, a shadow is a secondary quality. It requires a certain lighting to be applied to an object. For another example, consider the mass of an object. Weight is a secondary quality since, as a measurement of gravitational force, it varies depending on the distance to, and mass of, very massive objects like the Earth, as described by Newton's law. It could be thought that mass is intrinsic to an object, and thus a primary quality. In the context of relativity, the idea of mass quantifying an amount of matter requires caution. The relativistic mass varies for variously traveling observers; then there is the idea of rest mass or invariant mass (the magnitude of the energy-momentum 4-vector), basically a system's relativistic mass in its own rest frame of reference. (Note, however, that Aristotle drew a distinction between qualification and quantification; a thing's quality can vary in degree). Only an isolated system's invariant mass in relativity is the same as observed in variously traveling observers' rest frames, and conserved in reactions; moreover, a system's heat, including the energy of its massless particles such as photons, contributes to the system's invariant mass (indeed, otherwise even an isolated system's invariant mass would not be conserved in reactions); even a cloud of photons traveling in different directions has, as a whole, a rest frame and a rest energy equivalent to invariant mass. Thus, to treat rest mass (and by that stroke, rest energy) as an intrinsic quality distinctive of physical matter raises the question of what is to count as physical matter. Little of the invariant mass of a hadron (for example a proton or a neutron) consists in the invariant masses of its component quarks (in a proton, around 1%) apart from their gluon particle fields; most of it consists in the quantum chromodynamics binding energy of the (massless) gluons (see Quark#Mass).