[wim]

Well, it is distortion, because the image is not rendered in a rectilinear projection as we see it (more or less...) That a small part of the image is not affected by it doesn't change the overall phenomenon, does it?

I would only consider it distortion if it is not intended that way. If it was a rectilinear lens, yes, it would be distortion. Some fisheyes actually do show some distortion, although that tends to be hard to see. In that case it generally is pincushion distortion.

That lines through the optical center are straight is part of the way a fisheye renders, by design .