De jure (in Classical Latin de iure) is an expression that means "based on law", as contrasted with de facto, which means "in fact". De jure should not be confused with the French du jour, which translates to "of the day", as in soupe du jour.
The terms de jure and de facto are used instead of "in principle" and "in practice", respectively, when one is describing political situations. De jure is also translated as "by law". A practice may exist de facto, where for example the people obey a contract as though there were a law enforcing it yet there is no such law. A process known as "desuetude" may allow de facto practices to replace obsolete laws. On the other hand, practices may exist de jure and not be obeyed or observed by the people.
The phrase is often used in the context of (American) desegregation legislation. Codified segregation is de jure segregation. The more recent trend of minorities staying in the same areas due to socioeconomic factors is not legislated and is thus de facto segregation.