'Angel of the Lord' in the Bible

But an angel of the Lord opening the prison doors by night led them out and said,

And an angel of the Lord spoke to Philip, saying, Arise and go to the south, by the way that leads from Jerusalem to Gaza; this is a wilderness.

And behold, an angel of the Lord came, and a light shone in the building; and striking Peter on the side he awoke him saying, Arise quickly. And his chains fell off from his hands.

And an angel of the Lord instantly smote him because he gave not glory to God, and being eaten with worms he expired.