A way of remapping the surface of a sphere on a flat image, where the image has an aspect ratio of 1:2 (height:width). The equirectangular projection was used in map creation since it was invented about 100 A.D. by Marinus of Tyre.
In an equirectangular panoramic image all verticals stay vertical and all horizontals stay horizontal. Coordinates in the image relate linearily to pan and tilt angles in the real world. The poles (zenith and nadir) are located at the top respectively bottom edge and are stretched to the whole width of the image.
Equirectangular images are displayed by PTViewer, part of the Panorama Tools and some other viewers as spherical panorama.