'Angel of the Lord' in the Bible

But an angel of the Lord opened the prison doors during the night, and let them out.

And an angel of the Lord spoke to Philip, saying, "Arise and go on your way south, along the road that runs down from Jerusalem to Gaza, the desert way."

suddenly an angel of the Lord stood by him, and a light shone in the cell. Striking Peter on the side, he woke him saying, "Rise up quickly." At once the chains dropped from his hands.

Instantly an angel of the Lord smote him, because he had not given God the glory, and being eaten up by worms, he died.