Define Progressive

[kmack]

I like Genesis, Yes and ELP but how did that type of music get the "progressive" label? If you look at Websters, progressive is defined as:

pro•gres•sive \pre-"gre-siv\ adj
1 : of, relating to, or characterized by progress
2 : moving forward or onward : advancing
3 : increasing in extent or severity
4 : of or relating to political Progressives
5 : of, relating to, or constituting a verb form that expresses action in progress at the time of speaking or a time spoken.

So, why did "progressive" music get labled as progressive?