Chi (upper case Χ, lower case χ) is the 22nd letter of the Greek alphabet. In the system of Greek numerals it has a value of 600. The 'ch' in 'chi' is a voiceless palatal fricative. In front of "light" vowels (e, i, oi, ai, y) it is prounounced like the 'ch' in German 'ich' or some variants of 'h' in the English 'hew' or 'human'. In front of "dark" vowels (a, o, ou) and of consonants, it is pronounced like the 'ch' in German 'ach' or in Scottish 'loch'.

The upper-case letter Χ is used as the symbol for:

• The name of Jesus Christ in Christianity, as in X-mas etc.
  

The lower-case letter χ is used as the symbol for:

• The chi-square distribution in statistics.
  
• The electric susceptibility (sometimes with a subscript e) and magnetic susceptibility in physics.
  
• The chromatic number and the chromatic index (when shown with a prime mark or a subscript 1) of a graph in graph theory.
  
• The Euler characteristic in algebraic topology.
  
• The voiceless uvular fricative (IPA) in phonology.
  

Chi is the basis for the name Chiastic structure and the name of Chiasmus.