A relationship that holds between two images, A and B, where:
A depicts X;
B depicts Y;
X and Y are both of type T'
C is a 2 layer image consiting of layers A and B;
A and B are aligned in C according to a shared co-ordinate framework so that common features of X and Y are co-incident with each other.
Note: A and B may be 2D or 3D.
Examples include: the relationship between two channels collected simultaneously from a confocal microscope; the relationship between an image dpeicting X and a painted annotation layer that delineates regions of X; the relationship between the tracing of a neuron on an EM stack and the co-ordinate space of the stack; the relationship between two separately collected images that have been brought into register via some image registration software.