A more thorough explanation:

Definition: Promatertera magna is a Latin term used in civil law to refer to a great-great-great-aunt.

For example, if your father's grandmother had a sister, that sister would be your promatertera magna.

Another example would be if your mother's great-grandmother had a sister, that sister would also be your promatertera magna.

These examples illustrate how promatertera magna is used to describe a specific familial relationship. It is a term used to refer to a relative who is three generations removed from the speaker.