For a basic Xi meaning, look to the Greek alphabet. Xi is the 14th letter of this system. As a Greek letter, you can also find more than one Xi definition in the world of math and science. In particle physics, one Xi meaning is as a class of particles identified as Xi baryons or cascade particles. In the mathematical field known as set theory, a very broad Xi definition is as a symbol of the concept of a universal set, a set that encompasses all things including itself.