There are really two different definitions of asymmetric warfare, depending on who is doing the defining (from a military or political standpoint). One definition deals with tactics, such as what we know as guerilla warfare.

The other is used more politically, and refers to raw numbers rather than tactics. If a group is vastly overmatched (such as 5:1 or greater for instance), it is also considered asymmetrical.

They tie in to each other though. Generally, the second definition or condition leads to using the tactics in the first. Why they are separate schools of thought is beyond my comprehension.