Purity: Definition and Characteristics of Ḥayḍ
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 432:
Ḥayḍ is blood that is usually discharged from the uterus of women every month for a few days. A woman in menstruation is called a ‘ḥāʾiḍ’.
Most of the time, the blood of ḥayḍ is thick and warm, its colour is black or red, and it comes out with a little pressure and a burning sensation.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)