The term action in personam is used mostly in legal proceedings because Roman law heavily influenced our judicial system. Many terms used in law have their roots in Roman law, not only this and the Latin language is a cornerstone to those practicing law, not only in the US.
The term isn’t used in everyday language as there are perfect equivalents for it in every language. The fact that in the legislature, we, and so many other countries, still use Latin terminology or terms derived from Latin works as a testament to the impact the Roman Empire had on some essential aspects of the modern world.
But what is personam, or in personam? The term comes from the Latin “in personem”, meaning “against a person” and not another person, item, or possession. It refers to only one person, in particular, that is known or otherwise named.
Action in personam is a term used in judicial proceedings for actions brought against a person and not a property. Judicial proceeding in personam will be against the person rather than against the person's property or anyone else and anything else. In common law, it seeks the payment for a debt or damages incurred like the case can be for a divorce suit that involves real estate property as part of the assets, the lawsuit is against the person, not their house.
To put it more plainly, when a lawsuit is filed in court, it can either be made “in personam” or “in rem”, which means that it can either be against a person or anything else. This leads to the term action in personam being equivalent to “action brought against a particular person”. The concept comes from a legal interest that comes from an obligation and action between people. As a result, if a judgment is passed in personam it firstly affects the individual, and only after that does it affect the individual’s property and assets.