Amplitude: it's the distance from the horizontal axis to the extreme peak or trough (it's called the “power” of the cycle). Normally the amplitude is a function of its duration- the longer the cycle, the larger the swing.
Expressed in dollars, pips, or points, it's related to volatility.
Power of amplitude can be influenced greatly by exogenous, unpredictable events some of them anticipated in FXStreet's Sentiment Aggregator.
Because amplitude is considered a projection problem, the most reliable projections are made strictly on periodicity and phase.
Period: it's the distance between troughs. While the amplitude appears to change quickly at times, the period appears to change more slowly. The period often remains relatively constant and is an estimate based on immediate past price history.