A more thorough explanation:

The Institutes of Justinian is an elementary treatise on Roman law in four books. It is one of the four component parts of the Corpus Juris Civilis. The Institutes were written in the sixth century A.D. by the Byzantine emperor Justinian I. It served as a foundation for the study of Roman law and was used as a textbook in law schools throughout the Middle Ages.

• Coke's Institutes in four volumes (published in 1628) is a legal treatise or commentary.
• Gaius wrote an elementary treatise on Roman law in four books called the Institutes of Gaius in the second century A.D.
• Theophilus, a law professor at Constantinople, wrote a paraphrase of Justinian's Institutes in Greek in the sixth century A.D. called the Institutes of Theophilus.
• A person named in a will as heir, but under directions to pass the estate on to some other specified person (called the substitute) is also called an institute in civil law.
• The American Law Institute is an organization devoted to the study and improvement of the law.

The examples illustrate the different meanings of the word "institute" in law and legal education. The first three examples refer to different versions of the Institutes of Justinian, which was an important legal textbook in the Middle Ages. The fourth example shows how the word "institute" can be used in civil law to refer to a person named in a will. The fifth example shows how the word "institute" can be used to refer to an organization devoted to the study of law.