The interface has existence. It 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 others. We shall present herein an ontological work on the interface: by defining what it is and how it exists; and by discussing its relation to other entities. The interface will be 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 interfaces.