to be or become ceremonially or morally unclean, or to make something or someone unclean
This is the verb behind Israel's whole world of clean and unclean. At its heart it describes a change of state: a person, an object, a place, or the land moves from a condition fit to approach God and the community into a condition that is not. The same verb covers both sides of that change. It can mean to fall into uncleanness, as when touching a dead body or a person with a discharge makes someone unclean until evening. It can mean to put another person or thing into that state, to defile or pollute them. And in the laws of skin disease it carries a third, official use: a priest examines a case and pronounces the person unclean, a verdict that carries legal force in the camp.
The uncleanness in view is not the same as dirt or disease in our sense, and it is not always sin. Much of it is ordinary and unavoidable, a normal part of life such as childbirth, sexual activity, menstruation, or burying the dead. It simply meant a person could not enter the sanctuary or share in holy things until they were cleansed. The point was to keep what is holy and what is common from touching carelessly.
But the prophets stretch the word toward something heavier. Idolatry, bloodshed, and sexual sin defile the worshipper, the temple, and even the land itself, until the land is said to vomit out its people. Here the verb names a real moral stain that ritual washing cannot remove, only repentance and God's own cleansing. The single word holds both the routine impurity of daily life and the deep defilement of a people who have turned from God.
It is the direct opposite of being clean and being made clean. Where this word marks the slide into impurity, its counterpart marks the return to purity that lets a person draw near again.