Isaac signifies laughter, not from Sarah's laughter, which as yet had not happened, but from Abraham's past laughter, Genesis 17:17, and future joy in his son.
Related Commentaries of Genesis 17:19
Genesis 17:19
19 And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him.