What the lambda symbol means
Lambda (λ, capital Λ) is the 11th letter of the Greek alphabet. Unicode spells it "lamda", the modern Greek spelling, so try both spellings when you look it up in character tools.
In math, λ usually marks an eigenvalue (Av = λv), a Lagrange multiplier or the rate of a Poisson distribution. In computer science, the lambda calculus, created by Alonzo Church in the 1930s, gave its name to anonymous functions: Python writes them as lambda x: x * 2, and languages such as Java, C# and JavaScript write the same idea with an arrow.
In physics, λ is the symbol for wavelength (λ = c/f, covered on the wavelength symbol page) and for the decay constant of a radioactive isotope. Capital Λ is the cosmological constant in Einstein's equations of general relativity.
Lambda in gaming and LGBTQ history
Gamers know λ as the logo of the Half-Life series. The name and the logo point to the same physics: λ is the decay constant, which sets a radioactive material's half-life. That is why λ shows up so often in gamer tags, clan names and Steam profiles.
In 1970, the Gay Activists Alliance in New York chose lambda as its emblem, and it became one of the early symbols of the gay rights movement, later carried in names such as Lambda Legal.
Several characters look like lambda: ʎ (a turned y from phonetics), ⅄ (a turned sans-serif Y, see upside-down Y symbols) and ƛ, a barred lambda used in Americanist phonetic notation and some Indigenous languages of North America. The capital Λ also resembles the logic sign ∧ ("and"), which is a separate math operator.
How to type λ
Windows has no Alt code for λ. In Word, type 03BB and press Alt + X, or insert an equation and type \lambda followed by a space. On a Mac, open the Character Viewer (Control + Command + Space) and search "lambda", or switch to the Greek keyboard, where L types λ.
In LaTeX, use \lambda and \Lambda. In HTML, use λ and Λ. On phones, add the Greek keyboard or copy λ from this page. More letters are on the Greek letters page.