What is the Greek letter for 'Σ'?

The Greek letter 'Σ' represents the letter Sigma. In the Greek alphabet, Sigma is the eighteenth letter and is commonly associated with the English 'S' sound. It is also used in various mathematical contexts, particularly to denote summation, symbolizing the addition of a sequence of numbers.

The other choices represent different letters in the Greek alphabet. For example, Pi refers to the letter that corresponds with the 'P' sound, Tau corresponds to the 'T' sound, and Zeta corresponds with a different letter entirely, associated with the 'Z' sound. Each of these letters serves its unique function and has distinct uses across mathematical, scientific, and literary contexts, but they do not represent the letter 'Σ'. Therefore, Sigma is the correct answer for this question.