Definition: chapter book
Generally, a children's book that is long enough to be split into chapters, yet not long enough or deep enough to be identified as a novel, is known as a "chapter book." Chapter books are often illustrated, but not anywhere nearly as much as a picture book. Chapter books tend to be particularly popular with 7-10 year-olds as transitional books between beginning reader books and novels. Chapter books also tend to appeal to reluctant readers of all ages.
However, in the last few years, there has been a tendency on the part of many children and some schools and libraries to refer to any book with chapters as a chapter book, which has really confused the issue as to what a chapter book is. As far as I am concerned, there are two main types of chapter books: brief beginning reader books, generally for kids in grades 1-3 and slightly longer books (65 to no more than 200 pages) for more advanced readers in grades 2-5.