In physics, a dimensionless physical constant is a physical constant that is dimensionless, i.e. a pure number having no units attached and having a numerical value that is independent of whatever system of units may be used.^: 525 In aerodynamics for example, if one considers one particular airfoil, the Reynolds number value of the laminar–turbulent transition is one relevant dimensionless physical constant of the problem. However, it is strictly related to the particular problem: for example, it is related to the airfoil being considered and also to the type of fluid in which it moves.

On the other hand, the term fundamental physical constant is used to refer to some universal dimensionless constants. Perhaps the best-known example is the fine-structure constant, α, which has an approximate value of 1⁄137.036.^: 367 The correct use of the term fundamental physical constant should be restricted to the dimensionless universal physical constants that currently cannot be derived from any other source. This precise definition is the one that will be followed here.

However, the term fundamental physical constant has been sometimes used to refer to certain universal dimensioned physical constants, such as the speed of light c, vacuum permittivity ε₀, Planck constant h, and the gravitational constant G, that appear in the most basic theories of physics. NIST and CODATA sometimes used the term in this in the past.