Physical quantities which are constant and have no units.

1. Mathematical constants such as π (pi) and e (Euler's number)
2. Universal constants such as the speed of light in vacuum (c) and the gravitational constant (G)
3. Planck's constant (h)
4. Avogadro's number (N_A)
5. Boltzmann's constant (k)
6. Electron charge (e)
7. Fine structure constant (α)
8. Permittivity of free space (ε_0)
9. Permeability of free space (μ_0)
10. Stefan-Boltzmann constant (σ)

Dimensionless numbers