What is the meaning of χ?

a voiceless uvular fricative.

[χ]-fricated release of a plosive (e.g. [qᵡ], sometimes implying an affricate [q͡χ]); [χ]-coloring; or a weak, fleeting or epenthetic [χ].

a voiceless velar fricative (IPA [x]).

characteristic function (any meaning) or characteristic polynomial

Lower-case chi (χεῖ), the 22nd letter of the ancient Greek alphabet. It represented the voiceless aspirated velar plosive /kʰ/ and later the voiceless velar fricative /x/. It is preceded by φ and followed by ψ.

The lower case letter chi (χι), the 22nd letter of the modern Greek alphabet.

Source: wiktionary.org