The paper defines as "social signal" a communicative or informative signal that, either directly or indirectly, conveys information about social actions, social interactions, social emotions, social attitudes and social relationships. It proposes a conceptual definition of these social facts and some examples of relevant social signals.
Poggi, I., D'Errico, F. (2010). Cognitive Modelling of Human Social Signals. In Proceedings of SSPW 2010 – Social signal Processing Workshop, (pp.21-26). New Jersey : ACM, Sheridan Press [10.1145/1878116.1878124].