Opening in the skull of Amniotes, located between or next to to the postorbital and the squamosal bones.
In a Synapsid skull only one is present, located beneath the touching postorbital] and the squamosal bones.
In a Diapsid skull, two fenestrae are present, with the postorbital and the squamosal bones forming a bar in between the two. A derivative of a Diapsid skull whereby the lower of the two fenestrae has been lost, is called a Euryapsid skull.