What the alpha symbol means
Alpha (α, capital Α) is the first letter of the Greek alphabet, so it often stands for "first" or "beginning". The phrase "the Alpha and the Omega" from the Book of Revelation means the beginning and the end, which is why Α and Ω appear together in Christian art.
In statistics, α is the significance level of a test: the risk you accept of rejecting a null hypothesis that is actually true, very often 0.05. In physics, an alpha particle is a helium nucleus (two protons and two neutrons), and α also marks angular acceleration and the fine-structure constant, about 1/137. In finance, alpha is the return an investment earns above its benchmark.
You will also see alpha in software (an alpha release comes before the beta), in biology (the α-helix of proteins), in astronomy (α is usually the brightest star of a constellation, as in α Centauri), in image editing (the alpha channel stores transparency) and in slang for a leader type.
Alpha vs. look-alike characters
Not every α-shaped character is the Greek letter. ɑ is the Latin alpha of the International Phonetic Alphabet, used for the vowel in American "father". ⍺ comes from the APL programming language, and ∝ is the math sign for "proportional to", which looks like a stretched alpha. For Greek text and formulas, copy the first symbol on this page, α (U+03B1).
The capital Α looks exactly like the Latin A, but it is a different character, which matters for search, sorting and passwords. If you want a stylish alpha for a username, the math versions 𝛂 and 𝜶 stand out more, but screen readers may announce them oddly, so keep them out of important text.
How to type α
On Windows, hold Alt and type 224 on the number pad. In Microsoft Word, type 03B1 and press Alt + X, or insert an equation and type \alpha followed by a space. In Google Docs, open Insert > Special characters and search for "alpha".
Macs have no built-in Option shortcut for α. Press Control + Command + Space to open the Character Viewer and search "alpha", or add the Greek keyboard, where the A key types α. On iPhone and Android, add Greek as a keyboard language.
In LaTeX, use \alpha. There is no \Alpha command, because capital alpha looks the same as A. In HTML, write α or Α. The rest of the alphabet is on the Greek letters page, starting with alpha's partner, beta.