Bronzing is a phenomenon where tones in the print (especially grey tones, cyans and in blacks that are made up from a composite of colours), take on an iridescent appearance or a colour cast different to what it should be.
When viewed from an angle you might see metallic blotches on the parts of an image that are affected by bronzing, which often take on a green / bronze tone – hence the name. Bronzing occurs because of some of the ink laid on the paper is not properly absorbed into the coating- instead sitting on top of the page.
Sometimes a change of paper can help to alleviate the problem. Also using a Protective Spray such as the Hahnemühle Protective Spray can help.
Printers such as the Canon PRO-Series 12 colour Photographic printers use a Chroma Optimiser, which reduce this effect - giving a more visually pleasing result that is also more colour accurate.