Sigma Chi (ΣΧ) is one of the largest Greek-letter secret and social fraternities in North America with 244 active chapters in the United States and Canada and more than 300,000 initiates. Sigma Chi was founded on June 28, 1855 at Miami University in Oxford, Ohio when members split from Delta Kappa Epsilon. Sigma Chi has seven founding members: Benjamin Piatt Runkle, Thomas Cowan Bell, William Lewis Lockwood, Isaac M. Jordan, Daniel William Cooper, Franklin Howard Scobey, and James Parks Caldwell. Sigma Chi is a part of the Miami Triad, along with Beta Theta Pi and Phi Delta Theta.

According to the fraternity's constitution, "the purpose of the fraternity is to cultivate and maintain the high ideals of friendship, justice, and learning upon which Sigma Chi was founded."

The fraternity's official colors are blue and old gold, and its badge is a cross with emblems on each of its arms: crossed keys on the upper arm, an eagle's head on the left arm, seven gold stars and a pair of clasped hands on the lower arm, and a scroll on the right arm. In the center of the cross, on a black background, are the gold symbols for the Greek letters Sigma (Σ) and Chi (Χ). The left and right arms are connected.

Sigma (upper case Σ, lower case σ, lower case in word-final position ς; Greek σίγμα) is the eighteenth letter of the Greek alphabet, and carries the 'S' sound. In the system of Greek numerals it has a value of 200. When used at the end of a word, and the word is not all upper case, the final form (ς) is used, e.g. Ὀδυσσεύς (Odysseus) – note the two sigmas in the center of the name, and the word-final sigma at the end.

The shape and alphabetic position of Sigma is derived from Phoenician shin 𐤔 .

The name of sigma, according to one theory, may continue that of Phoenician Samekh. According to a different theory, its original name may have been "San" (the name today associated with another, obsolete letter), while "Sigma" was a Greek innovation that simply meant "hissing", based on a nominalization of a verb σίζω (sízō, from earlier *sig-jō, meaning 'I hiss').

The uppercase form of sigma was re-borrowed into the Latin alphabet to serve as the uppercase of modern esh (lowercase: ʃ).

In handwritten Greek during the Hellenistic period (4th and 3rd centuries BC), the epigraphic form of Σ was simplified into a C-like shape. It is also found on coins from the fourth century BC onwards. This became the universal standard form of Sigma during late antiquity and the Middle Ages. It is today known as lunate sigma (upper case Ϲ, lower case ϲ), because of its crescent-like shape.

Chi may refer to:

CHI may stand for: