As I have sometimes noted, the legal system of the United States is very conservative. The procedures, causes of action and much of the legal terminology dates back hundreds, perhaps thousands of years. Much of what we call "the law" in the United States is directly descendant from the English Common Law system. One very obvious indication of the conservative nature of the law practice is the persistent use of Latin terms, not just Latin derived but the actual terms in the Latin language. They no longer teach Latin to new lawyers but in most cases, when someone starts out in law school, they are immediately faced with a seeming deluge of Latin terms and phrases.
As genealogists researching back in time in any European country, we eventually find records written completely of partially in Latin. However, if you are researching court records, including probate, personal injury, criminal or any other type of action, you may still run into Latin. I can say from experience that after a while, the Latin phrases become so familiar I forget they are in a non-English language. The words and phrases have become part of my "English" vocabulary.
Here is the list with my perspective on the usage. A literal translation of the phrases usually does not help in understanding how the term is used in the law. In addition, some of these phrases have become so common that they have passed into ordinary usage. Sometimes the common use of the words differs from their technical, legal usage. I have no intention of listing every Latin legal term ever used. For anything not on this list see Black's Law Dictionary.
Here we go.
et al. -- literally "and others"
This is one of those terms that has passed into common usage. It is an abbreviation of et alii which translates as "and others." In the law it is used as a common abbreviation in the heading of legal cases filed with the court to avoid listing all of the parties every time a pleading is filed.
et cetera, etc. -- literally "and other things"
The abbreviation of this term is extremely common and its usage in legal cases clearly parallels its usage in common English speech. It now is usually used for "and so forth" or in a list to indicate that the whole list is not included.
et seq. -- literally "and the following things"
Another common adoption into commonly spoken English, the term in the law is used to show that the the following items (unwritten) are caused by the first one listed. It is also used by lawyers to include numbered lists, pages or sections after the first number is stated as in being used for "and the following sections (of the law). It is roughly the equivalent of "and so forth." It is an abbreviation of et sequens meaning "and the following ones."
et uxor or et ux. -- literally "and wife."
In the not too distant past and in some places today, the husband and the wife were considered to be one person and the husband was the one. This is not a commonly spoken term in English. It is not used too often even in legal documents.
a priori -- literally "from earlier"
In a legal sense this term is used to refer to the existence of one or more causitive events.
ab initio -- literally "from the beginning"
This term refers to the fact that some legally enforceable provision in a deed, contract or agreement is in force from the time the document was created.
ad hoc -- literally "for this"
In common usage, but from a legal standpoint is refers to an unsupported argument or one that has no foundation. In genealogy, most online family trees would be ad hoc creations.
post hoc, ergo propter hoc -- literally "after this, therefore because of this"
Sometimes used in common speech but more common in philosophy and law. This could be said of much of what passes for genealogical research online. More commonly, genealogists use the phrase "same name, same person." The phrase can refer to any unsubstantiated argument or proposition.
sui juris -- literally "of his own right"
The phrase is used to refer to someone who is legally competent to manage their own affairs.
in absentia -- literally "in absence"
Said of a legal action such as a trial, court hearing, sentencing or other proceeding conducted without the presence of a necessary party. Usually applied when a party has voluntarily failed to appear in a scheduled hearing or trial.
That should get you started. Here is link to a much longer list.