There are several ways to describe the style of progressive rock music---The Oxford Pocket English Dictionary says it is characterized by "classical influences, the use of keyboard instruments, and lengthy compositions." According to Princeton University's WordNet, the style of progressive rock is "associated with attempts to combine rock with jazz and other forms; intended for listening and not dancing." Lastly, The Oxford Companion to British History describes progressive rock's style as "complex compositions exploiting electro-acoustic sounds."
According to "The Music's All That Matters: A History of Progressive Rock," an article published in the academic humanities magazine "Notes," the origins of progressive rock are usually traced back to Britain's late-1960s psychedelic music scene, particularly from the late music of the Beatles. The Oxford Companion to British History credits "groups like Pink Floyd" for developing progressive rock in the late 1960s.
In addition to Pink Floyd, "The Music's All that Matters" describes the bands Yes, Jethro Tull, and Emerson, Lake & Palmer (ELP) as progressive rock acts that achieved significant commercial success during the 1970s. Encyclopedia Britannica also names British bands Genesis and King Crimson as examples of progressive rock groups.