Before Cornish Crosses came along (1950's) all the cockrels born to egg laying breeds/crosses were turned into meat chickens and raised to about 3 lbs live weight. A fryer was simply a young chicken around 2 lbs dressed weight. They were most often leghorn cockrels.
There is still a law on the books in Washington regulating the labeling of "fryers", the age of the birds, the origin, etc.
The meat chicken industry changed a lot and now you do not see the term very often, except to mean a smaller chicken than a roaster. Terms do vary by region. But generally today you will hear the term roaster or broiler instead of fryer.
With that said, I still sometimes see bags of feed labeled "broiler starter/finisher" and "fryer starter/finisher". There is no difference.