Our own alphabet runs out fast: a, b, x and y are already taken. So science borrows the Greek alphabet. The golden rule: a Greek letter has no fixed meaning. Each lesson tells you what it is. Here are the ones you’ll find on this site.

σ σ, sigma

Say it
“SIG-muh”.
It means
Its meaning depends on the lesson: the sigmoid in AI, the standard deviation in statistics, or a constant for radiated heat in physics.

Real example · in the lesson Logistic regression or neural network? Same neuron

σ(z) = 1/(1 + e⁻ᶻ): the sigmoid, which squashes any number between 0 and 1.

θ θ, theta

Say it
“THAY-tuh”.
It means
Almost always, an angle.

Real example · in the lesson How does a computer turn a photo into numbers?

cos θ = a·b / (‖a‖‖b‖): the angle between two arrows.

λ λ, lambda

Say it
“LAM-duh”.
It means
A number that stretches or shrinks something. In AI, how much large weights are penalised. In physics, often a wavelength.

Real example · in the lesson How does a computer turn a photo into numbers?

cos(x, λx) = 1 for λ > 0: stretching an arrow doesn’t change where it points.

ν ν, nu

Say it
“new”. It looks like a v, but it's Greek.
It means
In chemistry, how many molecules of each kind take part in a reaction. In particle physics, the neutrino.

Real example · in the lesson Can you make something from nothing? Conservation, no fear

Electron lepton number Lₑ (e⁻ and νₑ: +1; e⁺ and ν̄ₑ: −1).

τ τ, tau

Say it
“tow”, rhyming with cow.
It means
A characteristic time: how long something takes to heat up or cool down, or how long a particle lives on average.

Real example · in the lesson Joules, watts or calories? Energy and power without fear

T(t) = T₀ + (P/hA)(1 − e^(−t/τ)): τ sets the pace at which it heats up.

ε ε, epsilon

Say it
“EP-sih-lon”.
It means
Something very small. In floating point, the gap between 1 and the next number that fits. In heat physics, emissivity.

δ δ, small delta

Say it
“DEL-tuh”.
It means
A small error or change. Don't confuse it with capital Δ, which is any change.

Real example · in the lesson Why does the computer say 0.1 + 0.2 isn't 0.3?

fl(x) = x(1 + δ): when rounded, x picks up a relative error δ.

π π, pi

Say it
“pie”.
It means
π ≈ 3.14159: a circle's circumference divided by its diameter. In particle physics, also the pion.

μ μ, mu

Say it
“mew”.
It means
In front of a unit, the prefix micro: a millionth (1 µm = 0.000001 m). In statistics, the mean.

α α, alpha

Say it
“AL-fuh”.
It means
In physics, the alpha particle: a helium nucleus. In AI, often the learning rate.

Real example · in the lesson What is a wall made of? Atoms and elements without fear

In 1911, Rutherford, Geiger and Marsden fired α particles: helium nuclei, tiny and positively charged.

γ γ, gamma

Say it
“GAM-uh”.
It means
In particle physics, a photon: a packet of light.

Try it

Flip through the deck. Before you read the name, try to say what each letter is called.

The Greek deck

σ sigma · "SIG-muh"

Where you'll see it:

  • σ(z): the sigmoid, which squashes any number between 0 and 1 (logistic regression)
  • σ: how spread out some data are, the standard deviation (huge numbers)
  • σ: the Stefan–Boltzmann constant, in the heat a body radiates (energy)
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Challenge

You see σ in an AI lesson and σ in a physics one. Do they mean the same?
What is θ almost always?

Sources

  • Greek letters as quantity symbols: ISO 80000-2:2019, Quantities and units — Part 2: Mathematics (iso.org); IUPAC, Quantities, Units and Symbols in Physical Chemistry (“Green Book”), 3rd ed., 2007 (iupac.org).
  • The prefix micro (µ): BIPM, SI Brochure, 9th ed., 2019 (bipm.org).

How this cheat sheet was made: written with Claude’s help from the sources above and translated from the Spanish original. The examples and meanings come from the site’s lessons. The drawings are original.