Answer: the frequency of the wave.
In fact, the frequency of a wave is defined as:
[tex] f=\frac{1}{T} [/tex]
where T is the period of the wave. We see that frequency is inversely proportional to the period: therefore, if the period decreases, the frequency of the wave increases. (practically speaking, it means that the interval between two consecutive oscillations of the wave is decreasing, therefore the frequency (which is the number of oscillations per second) is increasing).
