The Greek letter X is also known as CHI, which is the 22nd letter of the Greek alphabet. It is a consonant and is pronounced like the 'ch' in the English word 'CHEESE'. It is often used in mathematics, physics, and engineering to represent various constants and variables. In chemistry, it is used to denote the atomic number of an element. In astronomy, it is used to represent the declination of a star. In linguistics, it is used to represent the voiceless velar fricative sound.