Name | Standard symbol | Definition | Named after | Field of application |
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Fine-structure constant |  |  | | quantum electrodynamics (QED) (coupling constant characterizing the strength of the electromagnetic interaction) |
Havnes parameter |  |  | O. Havnes | In dusty plasma physics, ratio of the total charge carried by the dust particles to the charge carried by the ions , with the number density of particles |
Helmholtz number |  |  | Hermann von Helmholtz | The most important parameter in duct acoustics. If is the dimensional frequency, then is the corresponding free field wavenumber and is the corresponding dimensionless frequency [7] |
Lundquist number | S |  | Stig Lundqvist | plasma physics (ratio of a resistive time to an Alfvén wave crossing time in a plasma) |
Perveance | K |  | | charged particle transport (measure of the strength of space charge in a charged particle beam) |
Pierce parameter |  |  | | Traveling wave tube |
Beta |  |  | | Plasma and fusion power. Ratio of plasma thermal pressure to magnetic pressure, controlling the level of turbulence in a magnetised plasma. |
Poisson's ratio |  |  | | elasticity (strain in transverse and longitudinal direction) |
Q factor | Q |  | | physics, engineering (Damping ratio of oscillator or resonator; energy stored versus energy lost) |
Relative density | RD |  | | hydrometers, material comparisons (ratio of density of a material to a reference material—usually water) |
Relative permeability |  |  | | magnetostatics (ratio of the permeability of a specific medium to free space) |
Relative permittivity |  |  | | electrostatics (ratio of capacitance of test capacitor with dielectric material versus vacuum) |
Specific gravity | SG | | | (same as Relative density) |
Stefan number | Ste |  | Josef Stefan | phase change, thermodynamics (ratio of sensible heat to latent heat) |
Strain |  |  | | materials science, elasticity (displacement between particles in the body relative to a reference length) |
Erlang |  |  | Agner Krarup Erlang | telephony (a measure of offered load on a telephone circuit) |