The term prog rock can be very vague at times sure, especially when calling certain bands that like pink floyd or pinback since they sound well different than certain others at the time especially if you compare them to King Crimson. I guess it depends on the person you talk to as well about such bands. Genre can get very crazy with certain songs and pieces exactly and speaking of that and even ELP certain songs can be one long piece or a segment of pieces forming one big song.