This term describes the process whereby the foliage of some cultivars are reverting back to the original foliage of one of their parents. This process is very common on the variegated 'Drummondii' cultivar of Norway Maple (Acer platanoides 'Drummondii').
The adjacent photograph shows this species with its crown outlined. The top-right section has reverted back to the normal green appearance of the Norway Maple and has also distorted the overall crown shape due to its faster growth.
It is essential to notice and prune out such branches if you wish to retain the variegated foliage of the cultivar. If you do not, the parental foliage will quickly outpace the growth of the variegated foliage and subsequently produce a normal green Norway Maple (this is happening to the tree located in the background, this tree will soon revert completely and is probably now not restorable through pruning).