Some minor revisions to the 1st paragraph of the "Definition" section. 

An obviously jumbled word somehow crept into the beginning of the 2nd sentence of the paragraph:  "The formula useoefficients of price elasticity...."  I put in a proposed revision.  

I also changed "quantity" to "quantity demanded" for the sake of clarity and completeness. It's crucial that the definition be as precise as possible in emphasizing that PED is concerned only with changes in quantity demanded.  (Elasticity is one of the most difficult concepts in micro for most beginning students and for other potential readers/users  WP's "customers"; they tend to get confused by the slightest ambiguity about which quantity we mean.  An ounce of emphasis here doesn't preclude such confusion, but it can help avoid a pound of it.)

In addition, I added "services" to the definition as a token reminder that PED, as well as the laws of supply and demand, apply to both goods and services.

The only other noteworthy revision was to the beginning of the last sentence in the first paragraph to place more emphasis on the PED for the most typical types of goods and services.  Although the Giffen and Veblen exceptions are interesting, they are almost negligibly rare anomalies compared to the overwhelming number of typical goods and services governed by the Law of Demand.  (That's why it merits the rare status of a "Law"!)  The exceptions deserve discussion eventually (in a later section), but I think this introductory paragraph would be strengthened by giving them only the briefest possible mention in passing here.  Instead, this paragraph should focus attention on the crux of the issue that readers need to remember (the "take-away"), namely:  because the demand for most goods and services obeys the Law of Demand, their PED coefficients will always be negative.  

Again, elasticity is one of the most difficult topics at the introductory level of microeconomics.  It doesn't serve this section or the article well to risk distracting students and other readers from the main point too early with such minor exceptions, however interesting they may be to the intellectually curious readers. Once we've made the main point as clear as possible, then devoting some time to the exceptions is much less likely to cause confusion or ambiguity.
  
I have one more suggested revision along this line for the paragraph, and I'll return to it as soon as time permits.

I made a few other copyedits were purely minor, e.g., deleting an occasional unnecessary word to polish the prose just a bit.