To effectively interact with others, one has to understand the context in which the interaction takes place. This understanding allows us to know how to act, how to interpret others' actions, and thus how to react. In this paper, we first present a basic formalism of the notion of context through the use of an ontology in order to integrate the new piece of information into a robot knowledge base in a coherent way. From there, we present a Context Reasoner integrated into the robot's knowledge base system allowing it to identify the context in which it is interacting but also to identify the current context of the surrounding humans. The effectiveness of this Context Reasoner and its underlying representation of the context is demonstrated in a simulated but dynamic situation where a robot observes an interaction between two humans. This work is an initial step to endow robots with the ability to understand the nature of interactions. We think that this basis will help to develop higher decisional processes able to adapt the robot's behaviors depending on the nature of the interaction.