From Latin, "against a thing." Concerning the status of a particular piece of property.
For instance, in-rem jurisdiction refers to the power of a court over an item of real or personal property. The "thing" over which the court has power may be a piece of land or even a marriage. Thus, a court with only in-rem jurisdiction may terminate a marriage or declare who owns a piece of land. In-rem jurisdiction is based on the location of the property and enforcement follows property rather than person.
Contrast: In personam