This is the knowledge that is shared, but not known to be shared.
This definition was taken from: Borovoy, R., Martin, F., and Resnick, M. (1999). Revealing common ground: Augmentation on the edges of interpretive communities. In Hoadley, C. and Roschelle, J., editors, Proceedings of the Computer Supported Collaborative Learning (CSCL), Palo Alto, California, USA. Stanford University, Lawrence Erlbaum Associates. Available from: http://llk.media.mit.edu/papers/archive/A05.HTM.