A lowercase Greek letter commonly used to represent efficiency in physics or as a variable in mathematical analysis.
Examples
Efficiency in physics equations
\eta = \frac{W_{out}}{W_{in}} \times 100\%
Viscosity coefficient in fluid dynamics
F = \eta A \frac{dv}{dx}
Dirichlet eta function in number theory
\eta(s) = \sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{n^s}