Rectilinear means that something is defined by or made up of straight lines. These straight lines are what really make it rectilinear. This is a general definition, and there are more specific definitions for photographers. In photography, rectilinear has two meanings. The first is that something relates to straight lines in some way, shape, or form. The second one refers to a lens correction.
This basically means that the lens corrected the photo so that straight lines on the subject would be straight in the picture. That’s all there really is to rectilinear in photography, and in general. This is a word that not many people would know, in either context. Isn’t it such a cool word to simply say that something is made up of or constrained by straight lines?