The speed of light in a vacuum is an exact universal physical constant. Under the International System of Units (SI), the meter is defined such that the speed of light is exactly 299,792,458 meters per second. It sets the maximum speed limit for matter and information transfer in the universe and features in Einstein's mass-energy equivalence: E = m c².

Planck's constant relates the energy carried by a photon to the frequency of its electromagnetic wave: E = h ν. Defined as an exact quantity in the 2019 redefinition of SI base units, it is the foundational quantum of action in quantum mechanics.

Avogadro's constant defines the number of constituent particles (usually atoms or molecules) contained within one mole of a substance. It bridges microscopic atomic mass units to macroscopic grams in stoichiometry and chemistry.

Appearing in Newton's law of universal gravitation F = G (m₁ m₂) / r² and Einstein's field equations of general relativity, the gravitational constant determines the strength of gravitational attraction between bodies with mass.

The Boltzmann constant relates the average relative thermal kinetic energy of particles in a gas with the thermodynamic temperature of the system. It connects macroscopic thermodynamics to microscopic statistical mechanics: PV = N k_B T.

The elementary charge represents the electric charge carried by a single proton (positive) or electron (negative). It serves as the fundamental unit of electric charge in electrodynamics, chemistry, and semiconductor electronics.