η (EN)

letter, symbol

letter “η”

1. the seventh letter of the Greek alphabet
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   In studying Greek texts, one must recognize the letter η, which represents a long 'e' sound.

symbol “η”

1. (physics) a symbol representing efficiency or viscosity
   The engineer calculated the efficiency η of the solar panel to determine its performance.
2. (mathematics) a symbol used to represent various mathematical concepts, such as the Dirichlet eta function
   In number theory, the eta function η (s) is important for understanding the distribution of prime numbers.