What is the meaning of ξ?

Extent of reaction

Gnarl

Lower-case xi (ξεῖ), the 14th letter of the ancient Greek alphabet. It represented the consonant cluster /ks/. It is preceded by ν and followed by ο.

The lower case letter ksi/xi (ξι), the 14th letter of the modern Greek alphabet.

Source: wiktionary.org