A more thorough explanation:

Connubium is a term used in Roman law that refers to the legal capacity to marry and the collection of rights that come with a marriage between individuals who have the capacity to marry. The word is derived from the Latin words "con," meaning "together," and "nubere," meaning "to marry."

For Roman citizens, connubium specifically refers to the right to intermarry with other Roman citizens and to contract a Roman marriage with all the unique forms and effects of marriage between Roman citizens. This includes the patria potestas, which is the lifelong control of the father over his children, a distinctive feature of the Roman legal system.

Other related terms include concubinatus, which refers to a non-marital sexual relationship, and justae nuptiae, which means "lawful marriage."

An example of connubium in action would be a Roman citizen marrying another Roman citizen and gaining all the legal rights and protections that come with that marriage, including the patria potestas over any children born to the couple.

On the other hand, if a Roman citizen were to enter into a non-marital sexual relationship with someone, that would be considered concubinatus and would not come with the same legal rights and protections as a lawful marriage under connubium.

These examples illustrate how connubium was a crucial concept in Roman law, defining the legal rights and protections that came with marriage and distinguishing it from other types of relationships.