I didn't know what a chapter book was until I joined SCBWI. As a kindergartener, my teacher used that word, but I thought it meant any book that was long enough to be broken down into chapters and was kind of like a "kids' word" for novel in the same way that I thought of "potty" as the kids' word for "toilet" or "choo choo" as the kids' word for "train." It didn't occur to me that that word was used just with young kids because young kids were the intended audience.