An informal amendment to the Constitution occurs when the interpretation of a part of the Constitution changes over time. Unlike a true, or formal, amendment, the informal amendment does not actually change the Constitution itself. Rather, the change comes in the form of exactly how the Constitution is interpreted on a social level.
Informal amendments occur either when the general public's or the judiciary's opinions on a Constitutional law change over time. One example of this is voting rights in the U.S. Originally, the vote was only granted to monied land holders. Over time, however, the vote was extended to all males, then all citizens over 21, then to all citizens over 18. In this instance, the Constitution itself was never changed, but the way citizens looked at it did.