barrel distortion
image distortion in photographs caused by inherent imperfections in the camera/lens optical system, resulting in an image where straight lines appear slightly bent, stronger near image borders, such that square objects appear rounded or "bloated"
en-US/ˈbæɹəl dɪˈstɔɹʃən/
en-GB/ˈbæɹəl dɪˈstɔːʃn/