"Frequency just describes the cycles per second, or Hertz, that the
Voltage is. Here in the US its 60 Hertz. It is constant. The Pi in the
formula is also a constant. "


The argument of the cosine function is the angle between voltage and
current in an AC load. It varies depending on the inductance and/or
capacitance of a particular load. The symbol used for the angle is
normally theta, not Pi as the OP stated.