impurity that defiles and must be kept apart, pictured most concretely by a woman during her monthly period
This word names a state of ritual impurity so strong that whatever carries it has to be set apart from ordinary life and from worship. Its most concrete picture is a woman during her menstrual period: for a fixed number of days she and anything she touches are treated as impure, not because she has done wrong, but because this is a condition that must be kept separate until it passes.
From that vivid picture the word stretches outward. It can name filthiness or defilement of any kind: an act so repellent it pollutes, the corruption of a whole land, or idols and ill-gotten wealth that must be thrown away. In each case the same idea carries over: something unclean that has to be expelled or held at a distance.
The word also anchors a fixed phrase, the water for impurity, the specially prepared water sprinkled on anyone defiled by contact with a dead body. Here it points not to the impurity itself but to the remedy for it, the water that removes the defilement.
Running through all of these is one thread: impurity understood as something to be separated off. The likely root sense has to do with driving away or keeping at a distance, which fits every use from the menstrual period to the polluted land to the cleansing water.