In the dictionary, "outlier" is defined as a person whose residence and place of business are at a distance, or, an outlier is something that is situated away from or classed differently from a main or related body.
In Outliers: The Story of Success, Malcolm Gladwell describes an outliers as people who "are invariably the beneficiaries of hidden advantages and extraordinary opportunities and cultural legacies that allow them to learn and work hard and make sense of the world in ways others cannot." Gladwell challenges the idea that successful people are simply smarter or more talented than others. He proposes that many successful people are "outside" this classification. He proposes that some people are successful because of hard work, unseen advantages, opportunities as well as cultural and historical determining factors. This is a book which focuses on the role of nurture (more than nature) in determining success.
Two of the examples in the book are Bill Gates and Bill Joy. Both men were born at a time when the computer industry was just getting on its feet. They happened to be in two of the few places in the country where they had access to computers and could log thousands of free hours where they spent this time working on programming. Had either been born/raised in some other part of the country, it is less likely that they would have developed the skills necessary to affect the computer industry to the degree that they did.
An "outlier," in the context of this book, is one whose success is partially a result of propitious historical factors.