Quantity	Expression
Length Length In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...  (R)
Mass Mass Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...  (M)

N-body units are a completely self-contained system of units used for N-body simulation

N-body simulation

An N-body simulation is a simulation of a dynamical system of particles, usually under the influence of physical forces, such as gravity . In cosmology, they are used to study processes of non-linear structure formation such as the process of forming galaxy filaments and galaxy halos from dark...

s of self gravitating systems in astrophysics. In this system, the base physical units are chosen so that the total mass, M, the gravitational constant

Gravitational constant

The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...

, G, and the virial radius, R, are normalised. The underlying assumption is that the system of N objects (stars) satisfies the virial theorem. The consequence of standard N-body units is that the velocity dispersion of the system is and that the dynamical -crossing- time scales as .
The first mention of standard N-body units was by Michel Hénon (1971).
They were taken up by Haldan Cohn (1979)
and later widely advertised and generalized by Douglas Heggie and Robert Mathieu (1986).