A more thorough explanation:

A forbidden degree is a type of prohibited degree under the category of degree. It refers to a degree of relationship that is so close that marriage between the individuals is not allowed by law. For example, siblings cannot marry each other because they are in a forbidden degree of relationship.

The law generally prohibits marriages between all persons who are lineally related and within the third civil-law degree of relationship. This means that aunt-nephew and uncle-niece relationships are also prohibited. The term "forbidden degree" is also known as "Levitical degree" because the Bible prohibits incest in Leviticus 18:6-18.

For instance, if a brother and sister want to get married, they cannot do so because they are in a forbidden degree of relationship. The law prohibits such marriages to prevent genetic abnormalities and to maintain social order.

Another example is that a father cannot marry his daughter because they are in a forbidden degree of relationship. This is because such relationships are considered taboo and morally wrong in most societies.