Is it permissible to address one's wife as "Umm Al-Mu'minin" (Mother of the Believers) out of endearment?

The fundamental principle regarding the assigning of names is permissibility, unless there is a religious text indicating the name is disliked or forbidden. There is no objection to changing a name or calling someone by a better name. It has been authenticated that the Messenger of Allah, peace and blessings be upon him, changed the names of some of his Companions and some of his wives. Undoubtedly, among the best names for females are those that correspond to the names of the Prophet’s daughters or his wives. An "Umm al-Mu'minin" (Mother of the Believers) is every woman with whom the Messenger of Allah, peace and blessings be upon him, contracted marriage and consummated it. Therefore, there is no harm in calling your wife "Umm al-Mu'minin."