An informal amendment to the United States Constitution is one that occurs through non-traditional processes such as "de facto" changes of law. These processes of amendment, which may be the result of either circumstantial social change or judicial review, are considered non-traditional because they are not specified in the Constitution as legitimate means by which amendments can be made.
As a result, informal amendments can be rather controversial, especially those made by judicial review. This is the process whereby law courts are able to pass judgment on whether a federal law is constitutional.
In the U.S., judicial review will typically be carried out by the Supreme Court, but not always. In fact, all courts at all levels are permitted to evaluate federal legislation against the Constitution. The majority of other countries with constitutions grant this power of judicial review only to a specific type of court.
The other process by which informal amendment occurs is more organic and therefore usually less controversial. As society changes, parts of the Constitution become obsolete or unfair, warranting alteration. A prime example of this occurred during the Industrial Revolution, when the right to vote was informally extended beyond white, male landowners. as specified in the Constitution, to the middle and working classes.