What is 18th letter of greek alphabet?

The 18th letter of the Greek alphabet is called "Sigma" (Σ, σ/ς). It is written as a capital letter Σ and a small letter σ when used in lower case. Sigma is pronounced as "si-g-ma" in English.

In the Greek numeral system, Sigma is used to represent the value of 200. In math, Sigma is commonly used in the summation notation, where it represents the sum of a series of values.

In ancient Greece, the symbol of Sigma was used to represent the first letter of the word "sumata" which means "totaled." It was also used in arithmetic and geometry for indicating the sum of numbers or angles.

Sigma is also commonly used in science, engineering, and statistical analysis, where it often represents the standard deviation, which is a measure of the amount of variation or spread in a set of data.

In certain fonts, the small letter sigma has two forms, one used at the beginning or middle of a word, called "sigma," and another used at the end of a word, called "final sigma." Final sigma looks like a small "c" with a horizontal top.