Regardless of the term used to designate them, Brain-Computer Interfaces are “Interfaces” between a user and a computer in the broad sense of the term. This paper aims to discuss how BCIs have been defined in the literature from the day the term was introduced by Jacques Vidal. From a Human-Computer Interaction perspective, we propose a new definition of Brain-Computer Interfaces as “any artificial system that transforms brain activity into input of a computer process”. As they are interfaces, their definition should not include the finality and objective of the system they are used to interact with. To illustrate this, we compare BCIs with other widely used Human-Computer Interfaces, and draw analogies in their conception and purpose.