Reckoning / Number / Greek alphabet
Number · 24 letters
Greek alphabet
Science ran out of Latin letters long ago. The Greek alphabet supplies the second set, and many letters have settled meanings: λ is a wavelength, ρ a density, Ω the ohm. In astronomy α, δ give a star’s place on the sky and λ, β its place along the ecliptic.
Name that letter
The letters
alpha
angles; alpha particle; angular acceleration; the fine-structure constant; right ascension (astronomy)
beta
angles; beta particle; speed as a fraction of light speed, v/c; ecliptic latitude
gamma
photon, gamma ray; Lorentz factor; ratio of heat capacities; Γ the gamma function
delta
Δ a change or difference; δ a small change, the Kronecker delta; declination (astronomy)
epsilon
permittivity ε₀; strain; a small quantity; obliquity of the ecliptic
zeta
the Riemann zeta function; damping ratio
eta
efficiency; dynamic viscosity
theta
angles, the polar angle; Θ thermodynamic temperature in some texts
iota
rarely used: it is too easily read as i
kappa
curvature; compressibility; thermal diffusivity in some fields
lambda
wavelength; decay constant; ecliptic longitude; Λ the cosmological constant
mu
the prefix micro (upright); permeability μ₀; coefficient of friction; mean
nu
frequency; neutrino; kinematic viscosity
xi
ξ a coordinate or random variable; Ξ a family of baryons
omicron
almost never used: it looks like o
pi
π the circle constant; Π a product of terms
rho
density; resistivity; charge density
sigma
Σ a sum; σ standard deviation, the Stefan–Boltzmann constant, conductivity, stress; ς only ends a word
tau
torque; time constant, mean life; shear stress
upsilon
rarely used: it is too close to u and v
phi
angles; latitude in navigation and geodesy; Φ magnetic flux; the golden ratio φ
chi
χ² statistic; magnetic susceptibility; the Euler characteristic χ of topology
psi
wave function
omega
ω angular frequency; Ω the ohm (unit symbol), solid angle, longitude of an orbit’s ascending node
Character names: The Unicode Standard: code charts and character names (Mathematical Operators, Greek, Miscellaneous Symbols). Unicode spells λ “lamda”, a spelling also used in modern Greek.
Look-alikes
Some symbols have their own Unicode characters that look exactly like Greek letters. Text that mixes them searches and sorts badly, so the standard asks for the plain letter.
| Character | Use instead | Note | ||
|---|---|---|---|---|
| µ | U+00B5 micro sign | μ | U+03BC greek small letter mu | The micro sign is kept only for compatibility with old character sets; Unicode maps it to the Greek small mu. Either way the prefix is printed upright: μm, not 𝜇m. |
| Ω | U+2126 ohm sign | Ω | U+03A9 greek capital letter omega | The ohm sign is canonically the same character as Greek capital omega; Unicode recommends using omega. |
| Å | U+212B angstrom sign | Å | U+00C5 latin capital letter a with ring above | The ångström sign is canonically the letter Å. |
| ∆ | U+2206 increment | Δ | U+0394 greek capital letter delta | A separate operator character; in text the Greek capital delta is normally used. |
A letter that stands for a quantity is italic: λ = 500 nm. A unit or prefix symbol is upright: μm, Ω. Writing μm in italics turns the prefix into a quantity, so the rule matters. Sources: BIPM SI Brochure, ISO 80000-2.