Why was Fatimah al-Zahra called al-Zahra? May Allah reward you with good.

Fatima al-Zahra is a title for Fatima, the daughter of the Messenger of Allah, may Allah bless him and grant him peace, as mentioned in books of سير (biographies) and تراجم (biographical dictionaries). No specific reason for this title has been mentioned. Linguistically, Al-Zahra means luminous whiteness and beauty of color.