Lambda Symbol (λ): Wavelength, Eigenvalues and LaTeX
July 3, 2026
The lambda symbol is λ in lowercase and Λ in uppercase. Lowercase λ means wavelength in physics, an eigenvalue in linear algebra, a rate in statistics, and an anonymous function in programming. Copy either below, or jump to how to type it.
| Character | Unicode | HTML entity | LaTeX | Windows alt code |
|---|---|---|---|---|
| λ lowercase lambda | U+03BB | λ | \lambda | — |
| Λ uppercase lambda | U+039B | Λ | \Lambda | — |
What the symbol means
| Where | Meaning |
|---|---|
| Physics | Wavelength, in metres |
| Linear algebra | Eigenvalue — the scaling factor of an eigenvector |
| Statistics | Rate parameter of the Poisson and exponential distributions |
| Nuclear physics | Decay constant, the probability of decay per unit time |
| Programming | An anonymous function, from Church's lambda calculus |
| Machine learning | Regularisation strength in ridge and lasso regression |
| Cosmology | Λ is the cosmological constant, as in ΛCDM |
λ as wavelength
Wavelength is the distance between successive peaks of a wave, and it is tied to frequency by the wave speed. For light in a vacuum that speed is c, which is why the two are inversely proportional — higher frequency means shorter wavelength.
λ as an eigenvalue
An eigenvector is a direction a matrix does not rotate — it only stretches it. λ is how much it stretches by. The eigenvalues are the roots of the characteristic polynomial, which is where the determinant condition comes from.
λ as a rate
In the Poisson distribution λ is the mean number of events per interval; in exponential decay it is the probability of decay per unit time. The same letter, and closely related ideas — the waiting time between Poisson events is exponentially distributed.
How to type it
Windows
- λ has no legacy alt code. In Word, type
03BBand pressAlt + X - Elsewhere, press
Windows + .and search “lambda” - For Λ, use
039BwithAlt + X
Mac
- Open Character Viewer with
Control + Command + Spaceand search “lambda” - Or enable the Greek keyboard, where λ sits on the
lkey
HTML
λorλfor λΛorΛfor Λ
In LaTeX
Both cases are single commands in math mode. In text — “lambda calculus”, “AWS Lambda” — write the word rather than the symbol.
In lambda-calculus typesetting, \lambda x . x needs explicit spacing: \lambda x .\, x. LaTeX treats the dot as punctuation and will not space it the way you expect.
Frequently asked questions
What does λ mean in physics?
Wavelength — the distance between two successive peaks of a wave, related to frequency by the wave speed.
What does lambda mean in programming?
An anonymous function, one defined inline without a name. The term comes from Church's lambda calculus, where λ binds a function's argument.
What is λ in machine learning?
The regularisation strength. A larger λ penalises large coefficients more heavily, trading training fit for generalisation.
See also the theta symbol, epsilon symbol and sigma symbol.
Try it free
Save hours on your next paper.
Turn handwritten derivations into submission-ready LaTeX in seconds, so the hours go into the analysis instead of the notation.
Convert your paper
Empowering students and researchers with AI-powered tools for academic writing.
Go to appContact us© 2026 Underleaf. All rights reserved.