xi; Ξ, ξ +

(Greek: the fourteenth letter of the Greek alphabet; Ξ, ξ)

xi (ksi)
The 14th letter of the Greek alphabet (Ξ, ξ), represented in the English alphabet as "x".
xi hyperon, xi particle
An isotopic spin multiplet of quasi-stable baryons with positive parity, spin 1/2, isotopic spin 1/2 hypercharge -1, and approximate mass 1318 MeV.
xi-minus particle
A xi hyperon that carries a unit negative charge designated Ξ-; also known as "cascade particle".

A hyperon is a comparatively massive baryon that may be unstable or partially stable and is short-lived.

A baryon is a subatomic particle belonging to a group that undergoes strong interactions, has a mass greater than or equal to that of the proton (a stable, positively charged elementary particle), and consists of three quarks.

You can see all of the letters of the Greek Alphabet on this page.