In Jewish folklore, Rahab is the name of a sea-demon, a dragon of the waters, the "ruler of the sea". Rahab is or was the angel of insolence and pride, responsible for shaking the waters and producing big waves; he is also responsible for the roaring of the sea. According to some sources, he was the guardian angel of Egypt, a position often designated to other angels such as Belial, Mastema, Samael and Uzza.
This name originally designated the primordial abyss, the water-dragon of darkness and chaos, and so comparable to Leviathan and Tiamat. Rahab later became a particular demon, inhabitant of the sea, especially associated with the Red Sea, in this case sometimes associated with Leviathan.
The difference between Rahab and Tannin is unclear in Jewish literature.
This name was also applied to Egypt, and the destruction of the Pharaoh after the exodus of the Israelites from that country, was compared with the slaying of Rahab, perhaps a late corruption of Rahab slaying the Pharaoh (the Red Sea drowning his troops).
For example, see Isaiah 51:9-11, where Rahab is connected with the sea serpent. In this passage the destruction of the Egyptians through the drying (parting) of the waters is compared to the myth of the destruction of Tiamat and the creation of the world. In this case it is the creation of Israel as a nation that is described, rather than the creation of the world, yet the comparisons are striking. Both creations involve water and a holy wind over the waters, and both involve a battle between order (YHVH) and chaos (the Egyptians, the watery abyss, or Tiamat).
Rahab is mentioned in the Talmud and the Old Testament, and its etymology is given as "noise", "tumult" and "arrogance".