فَالْحَيْضُ دَمٌ يَنْفِضُهُ رَحِمُ بَالِغَةٍ لَا دَاءَ بِهَا وَلَا حَبَلَ وَلَمْ تَبْلُغْ سِنَّ الْإِيَاسِ
Hayd is uterine blood of a mature female in the legally relevant state described by the text: not caused by disease, not during pregnancy, and before menopause. This definition separates Hayd from Istihadah.