to be clean or pure, to become clean, or to make something clean; used of ritual fitness, of literal cleanness, and of being freed from sin.
This word covers the whole idea of being clean and pure. Most of its uses come from the laws of Israel's worship, where being clean is not about dirt but about being fit to come near God and join the worshiping community. A person who has touched a dead body, recovered from a skin disease, or finished the time after childbirth is held back from worship until the set days pass and the washings are done, and only then does the law say the person is clean again.
The same word can name slightly different things, and the difference is usually who or what is doing the cleaning. It can describe a person or object simply becoming clean once the conditions are met. It can describe an actual cleansing, where water, fire, or a rite makes something clean. And in the laws about skin disease it has a special courtroom-like use: the priest does not cure the person, he examines and declares the person clean, the way a judge gives a verdict. Reading these apart keeps you from thinking the priest healed the disease when the text only says he pronounced the verdict.
The prophets and the Psalms take this everyday worship word and turn it toward the heart. To be cleansed there is to be freed from the guilt and stain of sin, something only God can do. So the same verb that washes a healed body in Leviticus is the one David prays God will use on his conscience, and the one through which God promises to wash a whole people clean from their idols and give them a new start.