Wikipedia
Najis
In Islamic law, najis are things or persons regarded as ritually unclean. According to Islam, there are two kinds of najis: the essential najis which cannot be cleaned and the unessential najis which become najis while in contact with another najis.
Contact with najis things brings a Muslim into a state of ritual impurity ( najāsa, in opposition to ṭahārah, ritual purity). Ritual purification is then required before religious duties such as regular prayers are performed.