Using the Sigma Symbol
Sigma is the eighteenth Greek letter, and its capital form has stood for summation since Euler used it in 1755. Σ before an expression means "add up every term": Σxᵢ is the total of a list of values, and Σ from i = 1 to n of i² is the sum of the first n squares. ### Letter or operator?
Unicode separates the Greek capital letter Σ (U+03A3) from the summation operator ∑ (U+2211). Maths renderers draw ∑ larger and stack its limits above and below, while the letter stays at capital height with limits as subscripts. Plain-text editors show them almost identically, which is why the distinction feels academic, but it matters for LaTeX (\Sigma versus \sum), for screen readers, which announce "sigma" versus "n-ary summation", and for the Mac shortcut Option + W, which gives you the operator rather than the letter. Use the letter when you mean a Greek letter, such as a sigma-algebra or the name of a fraternity, and the operator inside a summation in typeset maths. In a bio or a caption, either will look the same.
Lower-case σ and ς
The small sigma carries a different set of meanings. σ is the population standard deviation, the reason quality engineers talk about "six sigma" and physicists cite "five-sigma" discoveries. It is also the symbol for mechanical stress, surface charge density, the Stefan–Boltzmann constant and the Pauli matrices. Greek spelling adds a twist: σ appears at the start or middle of a word and ς only at the end, as in Οδυσσεύς. ### Writing sums in plain text
Without stacked limits, put the bounds after the sign in subscript-like notation or in words: Σᵢ₌₁ⁿ xᵢ, or "Σ x_i for i = 1 to n". Add a space between Σ and a multi-character expression, none before a single variable. ### Typing it
Windows has no Latin-1 Alt code, though the DOS-era Alt + 228 produces Σ in many programs, and Word turns 03A3 into Σ with Alt + X. Mac and phone users who want the real letter should enable the Greek keyboard and press Shift + S, with plain S giving σ and W giving ς. In HTML the entity is Σ; the operator has its own entity, ∑.
Σ in culture
The letter has a long life outside maths. The Greek-life world uses it in Sigma Chi, Sigma Nu and dozens of other chapter names, which is why it shows up in profile names alongside the delta symbol and the theta symbol. Internet slang borrowed "sigma" for a lone-wolf archetype, and the letter or the ∑ sign now punctuates memes and gaming handles in the same spirit. A well-known lens maker carries the name too, and because the letter renders as text everywhere it keeps its shape in usernames on Discord and TikTok. Set the rest of the name in the bold fonts generator if you want it to match the letter's heavy strokes.