In multi-color printing, a condition in which successively-printed colors (or images) are not positioned correctly with respect to each other, caused by a variety of different factors, from a change in the dimensions of the substrate to inaccuracies in platemaking, or any of a host of other causes. See Register.
'Out of register is also used to describe a condition of two-sided printing in which a sheet's back printing does not accurately line up with the face printing.