An open textbook is an openly-licensed textbook offered online by its author(s). The open license sets open textbooks apart from traditional textbooks by allowing users to read online, download, or print the book at no additional cost.
For a textbook to be considered open, it must be licensed in a way that grants a baseline set of rights to users that are less restrictive than its standard copyright. A license or list of permissions must be clearly stated by the author.
Generally, the minimum baseline rights allow users at least the following:
to use the textbook without compensating the author
to copy the textbook, with appropriate credit to the author
to distribute the textbook non-commercially
to shift the textbook into another format (such as digital or print)
Many authors also grant rights such as:
to add, remove or alter content in the textbook, often on the condition that derivative works must have the same license
to copy and distribute the textbook without giving credit to the author to use the textbook commercially
Excerpt from Open Access Textbooks.