The word "progressive" has been defined to mean "forward moving", "widening in scope", "changing in increments" and so on. Progressive music always looks forward, striving to be new and different, dissenting vigorously from the current musical establishment. Like all serious art, it is a challenge to the senses, and like all serious music, it requires active listening.

The term, coined by the British music press in the late sixties, was by the very early seventies solidly in use to describe bands such as Yes, Genesis, King Crimson and Emerson Lake and Palmer. One of the earliest known uses of the term “progressive rock” actually appears on the liner notes of the first Caravan LP released in 1968, where it states: “Caravan belong to a new breed of progressive rock groups, freeing themselves from the restricting conventions of pop music by using unusual time signatures and sophisticated harmonies. Their arrangements involve variations of tempo and dynamics of almost symphonic complexity”.

At that time if you used that term, it was very clear what type of band or music you were trying to describe. However as bands continued to develop and incorporate even more diverse musical elements and styles, the term came to be a “catch-all” phrase for any band who was working outside the Top-40 or pop norms of the day.

You will often see the term "progressive rock" used as a term intended to cover the same field as "progressive music". "Progressive rock" is a common phrase that is generally not meant to restrict the concept to rock-related examples. Most progressive music is based to some degree on: 70's Art Rock, Symphonic Rock, Canterbury, Psychedelic, Fusion, Krautrock, Classical Rock, Folk, Space Rock, RIO, and Zeuhl Music (for a definition of these terms, see GEPR or Phil’s Klime FAQ’s to R.M.P.

Progressive music is about sound itself and all the diversity which that implies. It involves lengthily works of electronics, solo piano, organ or guitar, sounds of nature, collage construction, and dense conceptual pieces, as well as more familiar sounds as band-oriented jams featuring the standard guitars, percussions, keyboards, and bass.

To sum up, progressive music incorporates:

Complex arrangements usually featuring intricate keyboard and guitar playing.

A mixture of loud passages, soft passages, and musical crescendos to add to the dynamics of the arrangements.  Extended instrumental solos, perhaps involving some improvisation.

Combines classical music's sense of space and monumental scope with rock's raw power and energy.

The often use of a Mellotron or string synth to simulate and orchestra backing

The inclusion of musical styles from other than a rock format (e.g. - classical, jazz, folk, avant-garde, blues, eastern music).

A blending of acoustic, electric and electronic instruments where each plays a vital role in translating the emotion of compositions which typically contain more than one mood.

A mixture of loud passages, soft passages, and musical crescendos to add to the dynamics of the arrangements.

Multi-movement compositions that may or may not return to a musical theme. In some cases the end section bear little resemblance to the first part of the song.

Compositions predominantly on the longish side, structured, but sometimes created from unrelated parts, i.e. Genesis’ “Supper’s Ready”. 

One of the better explanations of progressive rock can be found on the liner notes from the second Gentle Giant LP entitled “Acquiring The Taste”: It is our goal to expand the frontiers of contemporary popular music at the risk of being very unpopular. We have recorded each composition with one thought - that it should be unique, adventurous and fascinating. It has taken every shred of our combined musical and technical knowledge to achieve this. From the outset we have abandoned all preconceived thoughts on blatant commercialism. Instead we hope to give you something far more substantial and fulfilling”.