xi baryon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two baryons that contain two strange quarks and have a neutral or negative charge, masses of 2,573 and 2,586 times that of the electron, and mean lifetimes of 2.9 × 10⁻¹⁰ and 1.6 × 10⁻¹⁰ second, respectively.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics A subatomic hadron particle, denoted by Ξ, comprising three quarks: one up or down quark, and two heavier quarks.