International Bitterness Units (IBU)
Bitterness is a taste that is detected mostly by the back part of the tongue. It is a sensation that grows more pleasurable with age. The sensation of bitterness can be modified by other senses. Aroma, in particular, can enhance perceived bitterness.
Bitterness is an essential flavor characteristic of a beer. Taste testing will give an indication of the bitterness of a finished beer. However it is important for brewers to have a quantitative, objective measurement of the degree of bitterness of beer., hence the use of the International Bitterness Units (IBU) scale. From these results, they are able to decide whether any process changes are needed. Also they can choose to add or remove hops. In addition they may determine to add Isomerised Alpha Acid to adjust bitterness.
In the 1950s and ’60s, the brewing community banded together to create the International Bitterness Units (IBU) scale. The scale was the solution to an issue brewers had with the hops they were using, where alpha acids were decreasing between harvest and brewing.
A beer with a high IBU value is not necessarily an indication of intense hop flavor, which is a common misconception.
Sources: alchemyoverlord, Master Brewers Assoc. ASBC & Wikipedia