The correct answer to this open question is the following.
The term of office for federal judges is that they serve for life.
The logic behind this provision is the following.
Federal judges are appointed by the President of the United States, but this appointment has to be ratified by the United States Senate or upper chamber.
Federal judges serve for life because this gives them security on this difficult job and they feel no pressure at all to make a decision that could be unpopular. Federal judges have to resolve the most complicated cases in the US or the most controversial. That is why they have to feel confident about the decision they will make. Furthermore, they serve for life because they do not depend on the sympathy of one president, or be afraid that when a new one comes to power they could be removed.