Answered by Sidi Mostafa Azzam
How is najas defined (what are the different substances/material and conditions)?
Answer:
How najas (filth) is defined (i.e., the “intention” or “connotation” of najas) is, “something disgusting that prevents the validity of prayer when there is no dispensation” (al-Yaqut al-Nafis). As for what substances are najas (i.e., the “extension” or “denotation” of najas), refer to Reliance of the Traveller e14.1-5.
And Allah knows best.
Amman, Jordan