A cylindrical lens. This is the kind of lens generally used for extremely wide panoramic shots. It behaves like a normal perspective lens in the vertical direction, but it is non-linear in the horizontal dimension: a point on the film corresponds to a point in space in linear proportion to its angle to the camera, not to its straight-line distance from the center.
This allows up to 360 degree lenses in the horizontal dimension, with relatively little distortion. The distortion is not very apparent between two relatively nearby points on the film, but it becomes increasingly evident as you compare points widely spaced on the film.