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The eighteenth Greek letter, used for standard deviation and stress.
Also known as: Greek letter sigma, Lowercase sigma, Standard deviation symbol, σ
Lowercase sigma is the standard deviation in statistics — the letter behind 'six sigma' and 'a three-sigma result'. Its capital form Σ is a different thing entirely: an instruction to add a series up, which the catalogue records separately as the summation sign.
Is a letter of the Greek alphabet before it is a symbol: what it denotes is fixed by the field it appears in, not by the character itself. The uses below are the documented conventions, not competing definitions.
Contexts: Mathematics
In statistics, denotes the standard deviation of a population — the spread around the mean.
Contexts: Mathematics
In engineering, denotes mechanical stress, force per unit area.
Contexts: Mathematics
In physics, denotes electrical conductivity, or a scattering cross-section.
Contexts: Mathematics
In quality management, denotes a measure of process variation, as in the term six sigma.
Contexts: Mathematics
Has a capital form that is a symbol in its own right rather than a variable: Σ instructs the reader to sum a series, which is an operation rather than a quantity.
This is why the catalogue holds both records. σ names a quantity; Σ tells you to do something.
Contexts: Mathematics
Has a third written form, the final sigma ς, used only at the end of a Greek word and not used as a scientific symbol at all.
Contexts: Mathematics
A closed rounded loop with a horizontal stroke extending to the right from its top.
The capital of this letter is the summation sign. One names a quantity, the other instructs an operation, and readers routinely take the two for one symbol.