I have the time dependence of the electric field $A(t)$ written in terms of the amplitude $E(t)$ and phase $\phi(t)$ as

$$A(t) = E(t) \cos[\nu t + \phi(t)].$$

I am told that this "varies slowly in an optical period $2\pi/\nu$."

I'm a bit confused as to what this $2\pi/\nu$ is supposed to be. Is this the angular frequency? [Angular frequency is defined as](https://en.wikipedia.org/wiki/Angular_frequency)

$${\displaystyle \omega ={\frac {2\pi }{T}}={2\pi f},}$$

where $T$ is the period and $f$ is the ordinary frequency.

Furthermore, how can $\cos[\nu t + \phi(t)]$ be written so that the so-called "optical period" of $2\pi/\nu$ is clearer (to a novice)?