Eta

Eta (uppercase/lowercase Η η) is a letter of the Greek alphabet. In very early Greek writing it stood for the consonant sound "h", but in Classical Greek it stood for a long vowel "e". In Modern Greek, it is pronounced as "i". In the system of Greek numerals, it has a value of 8. Letters that came from it include the Roman H and Cyrillic И.

In mathematics, the lowercase η is used to represent Dirichlet eta function.^[1] The same letter is also used to represent conformal time in cosmology, efficiency in telecommunications and elasticity in economics.^[2]