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In situ is a Latin phrase for "on site" or "in position", and is used in science to mean that, especially regarding observations not made by using a laboratory to reproduce phenomena or relocate it for study. For example, in medicine, a body's reaction to a substance can be investigated by reproducing aspects of the body's internal conditions under a microscope, or alternately the observations might be attempted inside a person, i.e., "in situ".
In astronomy, the term is perhaps primarily used for space probes that measure phenomena at a particular location in space rather than gather EMR from a distance for interpretation, e.g., such as the phrase, in situ observation. Other uses include describing the location of origin of something, e.g., comparing the idea that the Earth's moon was captured after it formed, versus the notion that the Moon formed orbiting the Earth; the latter case can be stated as "the Moon was formed in situ".