In situ (a term used in biology) means to examine the phenomenon exactly in place where it occurs (without removing it in some special medium etc.). Usually means something intermediate between in vivo and in vitro. For example examing a cell within a whole organ intact and under perfusion may means in situ investigation. This is not obviously in vivo experiment because an animal is sacrificed, but it is not the same that to work with this cell alone (which may be a perfect case of in vitro experiment).