Latin. “Directed at a person.” The term in personam describes the fundamental nature of a legal action as focused against a person instead of a piece of property (in rem).
Black’s Law Dictionary, 2nd Edition
IN PERSONAM, IN REM. In the Roman law, from which they are taken, the expressions “in rem” and “in personam” were always opposed to one another, an act or proceeding in personam being one done or directed against or with reference to a specific person, while an act or proceeding in rem was one done or directed with reference to no specific person, and consequently against or with reference to all who it might concern, or “all the world.” The phrases were especially applied to actions; an action in personam being the remedy where a claim against a specific person arose out of an obligation, whether ex contractu or ex maleficio, while an action in rem was one brought for the assertion of a right of property, easement, status, etc., against one who denied or infringed it.
From this use of the terms, they have come to be applied to signify the antithesis of “available against a particular person,” and “available against the world at large.” Thus, jura in personam are rights primarily available against specific persons; jura in rem, rights only available against the world at large.
So a judgment or decree is said to be in rem when it binds third persons. Such is the sentence of a court of admiralty on a question of prize, or a decree of nullity or dissolution of marriage, or a decree of a court in a foreign county as to the status of a person domiciled there.
Lastly, the terms are sometimes used to signify that a judicial proceeding operates on a thing or a person. Thus, it is said the court of chancery that it acts in personam, and not in rem, meaning that its decrees operate by compelling defendants to do what they are ordered to do, and not by producing the effect directly.