Astigmatism
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A flat piece of glass. Also, any lens where the shape of the convex part matches the shape of the concave part. An example would be a flat piece of glass bent over a sphere.
irregularities
of Irregularity
Any polygon must have at least three sides. So a shape with only one pair of adjacent sides must be lenticular (shaped like a lens) else it is some very odd shape.
I do not think there is any shape that you would come across normally. One shape that I can think of is a bi-convex lens with a partial ridge. I am not entirely sure that the ridge counts as an edge, but that's my best offer.