The Collaborative International Dictionary of English v.0.48:

In situ \In si"tu\adv. & adj. [L.]
   1. In its natural or original position or place; in position;
      -- said specif., in geology, of a rock, soil, or fossil,
      when in the situation in which it was originally formed or
      deposited.
      [Webster 1913 Suppl.]

   2. (Med.) In its original position; not extending beyond the
      place of origin; localized and undisturbed; as, carcinoma
      in situ.
      [PJC]

WordNet (r) 3.0 (2006):

in situ
    adv 1: in the original or natural place or site; "carcinoma in
           situ"; "the archeologists left the pottery in place"
           [syn: in situ, in place]