This chapter defines the copyright public domain, as the basis for determining its categories, and what lies outside it. We argue that it can be seen as a cluster of related concepts with Wittgenstein-influenced ‘family resemblances’. Rejecting negative definitions, we define of the public domain positively in terms of the freedom (or negative liberty) to use ‘works’. ‘Freedom’ in this context is use that is not subject to permission from another, and therefore the touchstone of the copyright public domain is that it includes works (used to refer to all content and/or information resources) that are free to be used without permission. A similar touchstone has been used by scholars including Lessig, Litman, Boyle, Benkler and Deazley, but there are differences between them as to its scope. Our approach expands the definition’s scope to include copyright exceptions, to works under compulsory licences and to works under voluntary ‘neutral’ licences, but not to free access to works without free use. This leads to our definition of the copyright public domain. We then justify the definition, explain its advantages and explain the relationships between the public and private domains and our use of ‘commons’ and ‘public rights’.