M0

m0 is a symbolic label used in several disciplines to denote a baseline, reference, or invariant quantity. Because of its generic nature, it has no single universal meaning; context determines its interpretation.

In physics and relativity, m0 commonly denotes the rest mass of a particle, the intrinsic mass that does not change with its motion. The total energy is E = γ m0 c^2 and momentum p = γ m0 v, where γ is the Lorentz factor. The symbol m0 is frequently contrasted with the concept of relativistic or moving mass in older texts. It is a fundamental parameter in equations describing energy, momentum, and invariant mass.

In macroeconomics and financial statistics, M0 (capital M) denotes the monetary base or base money: currency in circulation plus banks’ reserves held at the central bank. It is a narrow money measure that serves as the foundation for broader aggregates such as M1 and M2, and is closely watched as an indicator of monetary policy stance and liquidity conditions.

In astronomy, M0 is used as a spectral classification marker for a star of spectral type M0, often written as M0V for a main-sequence star. It indicates a relatively cool, red star with characteristic absorption features that distinguish it from hotter stellar classes.

The capitalization of the term matters: m0 typically appears in lowercase when referring to mass in physics, whereas M0 is standard for the monetary base and for certain astronomical classifications. As with many technical symbols, field conventions determine its precise meaning, so context is essential to avoid confusion.