Like most greek letters, it's used for a lot of things in math and science, although I'm not sure if frequency is one of them. It's most often used for wavelength, radioactive decay (half-life), eigenvalues, linear charge densities, and lagrange multipliers. I'm sure it has other uses.
Sorry, not frequency, indeed it is wavelength, it was something with waves
I love greek symbols. Beta is my favourite ß. Fuck you Omega, nobody likes you! Ω