Phil Taylor has a good piece about the nature of "choking" over at SI.com. Specifically, he addresses whether or not the Yankees choked in the ALCS this year:

We need to get something straight about choking, which is merely the most misunderstood concept in all of sports. It has become a catch-all term, applied to any player or team who: A) blows a huge lead; B) fails in a crucial moment; or C) loses an important game more than once.

...But not everyone who fails when we expect them to succeed is a choker. In fact, most of them aren't. The Yankees are the latest team to have the tag slapped on them unfairly, a result of their unprecedented collapse against the Boston Red Sox in the ALCS. There are any number of words that accurately describe the Yanks' failure -- "humiliating" comes to mind -- but "choking" is not one of them. A choker is a player or team who loses because the pressure of the moment adversely affects their performance. There is no way that could logically be said of the Yankees. They have thrived under pressure so often that it's absurd to think that they suddenly crumbled because of the magnitude of the moment.