What does off-nadir mean?

Answer 1

Off-nadir is a Hebrew idiom and the meaning is "something very rare." In remote sensing terms, the term 'off-nadir' (or 'look angle') is used to refer to any point that is not directly below a sensor, but off to an angle in any direction.

Answer 2

For all practical purposes, nadir is direction of Earth's center. For instance, when using sensors or cameras to look at the Earth's surface from above, "nadir pointing" means that these devices are directed straight down. "Off nadir" indicates directions that point at the Earth's surface, but away from straight down. So, if one were to photograph a forest from an aircraft with the camera pointing straight down, the center of the resulting photo will be at nadir, while the rest of the photo will record the same forest off nadir (and increasingly so the farther from the center of the photo).