Diamond open access refers to open access journals that are free for readers to access and for authors to publish in. These journals are often community-driven and supported by institutions or by national or regional infrastructure. Many journals in the Open Library of Humanities or UC's eScholarship Publishing are "diamond".
Another model to accomplish Diamond Open Access is Subscribe-to-Open (S2O). Under this model libraries keep subscribing to a publisher's portfolio and in exchange the publishers makes all articles available open access at no cost for the author.