Open textbook is a textbook licensed under an open copyright license and made available online to be freely used by students, teachers, and members of the public. They are available for free as online versions and in a variety of file formats (e.g., for eReaders, editable files like XML and HTML), and as low-cost printed versions, should students or faculty opt for these. Open textbooks are a way to significantly reduce student textbook costs while giving instructors the flexibility to reformat and customize their course material. They are an affordable, flexible alternative to traditionally-published textbooks.