mass

Mass is the quality of an object that determines its gravity and also determines the degree of acceleration resulting from a force on the object. The term is also used for a measure of this quality associated with such an object. It is the quality that determines an object's weight when it is within a gravitational field, e.g., on the Earth's surface.

Mass was once considered an absolutely conserved value, but famously, the equation e=mc² adjusted that idea: mass is conserved in many "everyday" cases, but is considered a form or aspect of energy, the latter of which is conserved universally.

The above gravitational and acceleration/inertia effects are found to be proportional, suggesting they both result from the same quality. This is, in fact, tested with experiments measuring the degree to which this proportion holds, and no credible discrepancy has been found. (These two apparently-equivalent qualities are termed the inertial mass and the gravitational mass.)

Relativity has consequences regarding the meaning of the term mass. For an object at rest, the concept is straightforward, but from a frame of reference in which the object is moving, acceleration also depends upon the speed at which it is moving, as if the moving object's mass were greater. The term rest mass is clear, and currently, it is common to treat the term mass as rest mass, and call the apparent quality of such a moving object a relativistic mass (or else avoid referring to it). At one time, this consensus wasn't as clear and some earlier writing may treat the term mass as this relativistic mass, and the point of this is you must sometimes assure you know which meaning is intended.

Mass (taken as rest mass) makes the equation e=mc² true when the object is at rest, and otherwise does not include the effects of its motion (relativistic kinetic energy). If m were to be taken as the relativistic mass, then the equation would include both its rest energy and the effects of its motion. One means of describing the impossibility of accelerating an object to the speed of light is that its relativistic mass would be pushed toward infinity as you near that speed.

The term dynamic mass refers to the mass of a body determined through dynamics, e.g., using orbital periods. This requires knowledge (at minimum) of either the distance from Earth or the mass of another object within the gravitationally-bound system.

The term point mass refers to a modeling an object's mass as being at a single point. This simplifies problems and can be a very useful approximation if distances between objects are large compared to their size and/or if an object is close to being spherically symmetric.

Among the units of mass used in astrophysics:

A mass number is a count of nucleons in a nucleus, atom or molecule. Nucleons (i.e., protons and neutrons) have roughly the same mass and the mass number is an approximation of the mass of the particle in daltons.

The term mass can be used to mean (roughly) substance, i.e., something that has mass, in phrases like mass transfer, mass loading, and coronal mass ejection.