'Half brother' in the Bible

It happened afterwards that Absalom the son of David had a beautiful sister whose name was Tamar, and Amnon [her half brother] the son of David was in love with her.

He said to Amnon, “Why are you, the king’s son, so depressed morning after morning? Will you not tell me?” And Amnon said to him, “I am in love with Tamar, my [half-] brother Absalom’s sister.”