The Hebrew word niydah means removal, impurity, or something cast away. It is closely related to the concept of menstrual impurity (niddah, H5079) but carries a broader sense of anything that is removed or set apart due to uncleanness. The word conveys separation from the community or from God's presence due to defilement.
In the Old Testament, niydah underscores the holiness of God and the need for His people to maintain ritual and moral purity. The concept of removal or separation due to impurity points to the seriousness of sin — it creates distance between the creature and the Creator. Prophetically, Israel's idolatry is compared to impurity that must be cleansed. This imagery finds its ultimate resolution in the promise of a fountain opened for cleansing from sin and impurity (Zechariah 13:1).