In Weapons of Math Destruction, Cathy O'Neil identifies opacity as one of the characteristics of a WMD, a mathematical model that has damaging effects on people's lives. These mathematical models may be opaque in several different ways. Those it affects may not understand how it works. Those who operate it may be similarly ignorant. Finally, the people it affects may not even be aware that the WMD is operating at all.
An example of this is the way we are sorted into tribes based on our use of social media or our buying patterns on Amazon. This is done on the basis of complex mathematical models which few understand, and even those who are aware that they are being shown advertisements or offered financial products based on their internet usage generally do not know how these results are generated.
Transparency is the opposite of opacity. It means that a mathematical model is not a Weapon of Math Destruction according to O'Neil's definition, because those affected by it are aware of its existence and can challenge its conclusions. O'Neil regards the college ranking system as a typically opaque WMD. However, when the Obama administration failed to reform the system, the Education Department released extensive data about colleges on its website so students could make choices without reference to rankings. The Education Department's website is a good example of transparency, since the data is clear and is equally available to all. If the colleges want to dispute any of the data, they are free to do so.