Frequency of the wave in frequency domain
If you're working with a 2 dimensional data set, you need to do a 2D FFT on it (which can be dome using many 1D FFTs: doing all the rows and then all the columns, or if you prefer, the columns and then the rows).

The 2D FFT gives you a 2D array of complex numbers.  Horizontal waves will give peaks on the horizontal axis.  Vertical ones will give peaks on the vertical axis, and diagonal waves will give peaks away from both axes.