"Agile" is an umbrella term coined in 2001. It refers to a complete category of methods, including Extreme Programming, Scrum, Feature-Driven Development, DSDM, and so on.
Although you can "be agile", there is no "Agile" method to practice. "Agile" refers to any of the agile methods.
James Shore, coauthor of <a href="http://www.amazon.com/Art-Agile-Development-James-Shore/dp/0596527675" target="_blank" rel="nofollow">The Art of Agile Development</a>. Website and blog at <a href="http://www.jamesshore.com" target="_blank" rel="nofollow">http://www.jamesshore.com</A> .