All of the patterns a wave in AC follows are called waveforms, a plot of a chosen quantity against time.

The instantaneous value is the value at any point of the graph of this waveform.
Instantaneous values are always denoted using small letters.

A cycle is a portion of a waveform which when repeated makes up the entire waveform. When the wave goes from a point until it comes back to that same point again.

The Time Period () of an alternating quantity is the time taken to complete one cycle i.e. this quantity repeats every T time

The Frequency of a waveform is the inverse of the period, the cycles completed in an amount of time
The Amplitude is the maximum value (positive or negative) aka the highest peak/the lowest valley reached by an alternating quantity during its cycle.