The interface is more than an assemblage of processes and effects; it is more than a pure relational instance. The interface can be identified as an entity in itself and in relation to other entities. In this article, the author presents an ontological work on the interface, defining what it is and how it exists, and discussing its relation to other entities. The interface is defined both abstractly as a mediation complex—by identifying characteristics present in its multiple instantiations—and in relation to other entities—by observing the case of the human-computer interface.