In situ is a Latin term, which means at the site, or in position. It is used in different fields of science, with slightly different meanings. Examples include:
In medicine, it usually means at the position where it is in the body. When talking about cancer a carcinoma in situ is an erly stage of cancer, that has not spread, and that is usually easy to treat.
In physics, an in usitu measurement' is a measurement in place.
In chemistry it means that the (usually very reactive) mixture is directly used.
In archeology it means that the items are still at the position where they were found originally.