In historical geology, this is the name given to the ocean which existed between Africa/Europe and North America prior to the formation of Pangea. This is essentially the previous incarnation of the Atlantic Ocean. The closing of the Iapetus led to the collision between North America and Africa which resulted in the Appalachian Orogeny (~ 400mya).

The rifting between Africa and North America ~ 250 mya opened the modern Atlantic.

Little is known about the geography of the Iapetus Ocean, since it was almost completely destroyed by subduction as Pangea formed. However, some island arcs were accreted onto North America, and can be found as heavily metamorphosed exotic terrains within the Appalachians.