(roofing) A moderate deformation or buckling of sheet material, particularly common with flat sheet metal surfaces. Typically caused by uneven stresses at the fastening points. This terminology also refers to the popping sound made when pressure is applied to the deformed sheet forcing the deformation in the opposite direction.
(manufacturing) The movement of a perceived flat surface across its intended plane due to the application of a shear force. Caused by unequal stretching of the material inside the surface of the panel while retaining the perimeter. This term refers to the technique used to operate an oil can or an "oiler" whereas the user inverts the oiler and holds it between the index and middle finger while applying pressure to the bottom with their thumb to ultimately pump oil out of the spout. This phenomenon has additional exposure in the home canning environment where during the cooling process the lid of the jar has negative atmospheric pressure sufficient to pull or "pop" the lid center down assuring the lid has sealed to the jar. Membrane buttons on electronic devices give this sensation as well.