Enormity has a few meanings. It’s sometimes used to refer to (1) extreme wickedness, or (2) a monstrous offense or evil. The word comes from the French énormité, which mean irregularity, and these more extreme senses developed in English a century or two after the word entered the language.1 But enormity is also frequently used interchangeably with enormousness, which means the quality of being great in size, number, or degree. This is not wrong, nor is it a new development; the OED lists examples of enormity used this way from as far back as the late 18th century, and plenty more examples are easily found in historical searches. 2
The use of enormity as a synonym of enormousness peeves some people, and it is commonly called an error in writing on these matters. But trying to preserve the old senses of enormity is a losing battle given the word’s relative rarity and its similarity in sound to enormous, which unquestionably has to do with largeness and abundance. Plus, when we search the web for recent instances of enormity, it is clear that the word is almost always a synonym of enormousness in recent popular usage. Of course, people who don’t want to let go of precious language peeves tend to scoff at popular usage, but English is made by English speakers, and this issue has been settled in the minds of most English speakers.
In any case, for careful writers who wish not to cause confusion, avoiding enormity might be the best option for now. There are plenty of good synonyms to use instead.
The use of enormity to mean enormousness is rife in all types of writing in this century. Here are a few examples chosen at random out of a great abundance:
The enormity of the news that bin Laden had been killed left Wheaton College graduate student Meghan Cahill wrestling with her emotions Monday. [Chicago Tribune]
We are overwhelmed because we recognize the enormity of the problem but have no clear sense of what can be done. [Living in Denial, Kari Marie Norgaard]
Although fully subscribing to this aim, it is unclear whether the leaders of the time fully understood the enormity of accomplishing such a task. [The Encyclopedia of Applied Linguistics]
We do understand that it is already a Herculean task to conceptualize the enormity of a three-dimensional, seemingly infinite material universe from your perspective. [The Cosmos of Soul, Patricia Cori]
Mr Smith said he was rushed into theatre so quickly after being told he would receive Mr Rogers’ organs, he did not have time to consider the enormity of it. [BBC News]
In a century, as the population of Delhi has grown from around 200,000 to its current enormity, the ruins and the villages now stand not in open countryside, but atop roundabouts. [Newsweek on Daily Beast]
Of course, counterexamples—instances of enormity used as a synonym of wickedness—are easily found, but they are not as common as examples like the above:
And many others cried, too, sickened by the death toll, the enormity of almost 10 years of fear, death and terror. [Washington Post]
Only after Germans had appreciated and digested the enormity of their Nazi past … could they begin to live with it. [Postwar: A History, Tony Judt]
A refusal to grasp the enormity of the crime being committed continued for a long time. [Telegraph]
And in many such instances, it’s difficult to tell whether the writer means enormity in the wickedness sense or the enormousness sense, as the word often comes up in reference to enormous wickednesses.