There is no “hard” definition of Open Content, it just happens to be one of the most used terms. You may hear other people refer to it as creative commons, open license, open access, open educational resources, or free content.
Of course, all these terms technically mean different things. But for the most part, people are generally referring to content that is free to use without permission.
Open Content is said to exist under an open license. A Creative Commons license is one such example. Also known as a CC license, Creative Commons gives free access and distribution to an otherwise copyrighted work. Usually the author of the piece of content grants a CC license.