the 18th letter of the Greek alphabet
"The professor wrote the symbol sigma on the board to represent the variable in our equation."
The eighteenth letter of the Classical and Modern Greek alphabets, the twentieth letter of Old and Ancient.
"The professor asked us to identify which Greek symbol represented the sound in our new vocabulary word, and we pointed out that sigma was the correct answer."
In plain English: Sigma is the 18th letter of the Greek alphabet that scientists use to represent specific values in math and science.
"The professor wrote the Greek letter sigma on the whiteboard to introduce the summation formula."
Usage: Sigma refers to the 18th letter of the modern Greek alphabet or the 20th in ancient scripts; it is not a mathematical term for standard deviation unless specified as such. Use this word only when discussing alphabets, geometry involving specific angles, or statistical notation where context clarifies its meaning.
The word sigma comes from Ancient Greek, where it originally referred to a letter of their alphabet. It is likely derived from a root meaning "to hiss," reflecting the sound the character makes when pronounced.