He's talking about plane waves - i've uploaded the page as an image:
'Waves, however, behave quite differently. The right portion of Fig. 6.2 shows the effect of openings of various sizes on a plane wave incident from the left.
In the top right portion of the figure, the aperture is many times larger than the wavelength of the wave, and the wave passes through the opening without significant change. But, if you reduce the size of the opening so that the size of the aperture is similar to the wavelength of the wave, the portion of the wave that passes through the opening is no longer a plane wave.
As you can see in the center portion of the figure, the surfaces of constant phase are somewhat curved, and the wave spreads out after passing through the opening. And, if you reduce the size of the opening even more, such that the aperture is smaller than the wavelength of the wave, the wavefront curvature and spreading out of the wave becomes even greater. This effect is called “diffraction”.
He's talking about curvature of a plane wave - but that can be a cylindrical sort of curvature or a spherical one. So i was wondering which it was.