A loose leaf (also loose leaf paper, filler paper or refill paper) is a piece of paper of any kind that is not bound in place, or available on a continuous roll, and may be punched so as to be organized in a ring binder. Loose leaf paper may be sold as free sheets, or made up into notepads, where perforations or glue allow them to be removed easily. "Leaf" in many languages refers to a sheet or page of paper, as in Folio, as in feuille de papier (French), hoja de papel (Spanish), foglio di carta (Italian), and ルーズリーフ (Japanese, "ru-zu ri-fu").

"Loose leaf" describes any kind of paper or book that is available in single sheets, unbound. Its "leaves", or sheets, are "loose" and not bound in notebook or book form. In North America, some textbooks are sold with prepunched holes and perforated pages, so that users can remove the pages and store them in a typical 3-ring binder. This helps in that the user is therefore able to carry only the part of book that is in use with them, without needing to carry the whole book.

Main paper sizes are the letter-size system mainly used in North America and the ISO system used in the rest of the world. US companies such as Staples and Office Depot manufacture and sell letter-size loose leaf products in their retail stores. When it comes to ISO-sized loose leaf systems, since Japanese companies (e.g. Kokuyo, Maruman, MUJI, King Jim) are major designers and manufacturers of ISO-size loose leaf systems, whose products are sold internationally, corresponding Japanese terms will be included in parentheses throughout this article.