1. In medieval jurisprudence, a body or collection of various laws peculiar to a given nation or people; not a code in the modern sense, but an aggregation or collection of laws not codified or systematized.
2. In modern American and British jurisprudence, a system or body of laws, written or unwritten, or so much thereof as may be applicable to a particular case or question, considered as being local or peculiar to a given state, country, or jurisdiction, or as being different from the laws or rules relating to the same subject-matter which prevail in some other place.
3. In old English law, a body or collection of laws, and particularly the Roman or civil law.
4. Lex is used in a purely juridical sense, law, and not also right; while jus has an ethical as well as a juridical meaning, not only law, but right.
5. Other specific meanings of the word in Roman jurisprudence were as follows:

Positive law, as opposed to natural.

That system of law that descended from the Twelve Tables, and formed the basis of all the Roman law.

The terms of a private covenant; the condition of an obligation.

A form of words prescribed to be used upon particular occasions.

Law.

Motto of San Francisco Law School, San Francisco, California, USA.