In The Lord of the Rings: The Return of the King, the pass where Frodo enters Mordor is Cirith Ungol.
Cirith Ungol is a mountain pass in the Ephel Dúath, the range forming Mordor’s western boundary. Gollum leads Frodo and Sam toward it because he believes the route will avoid the heavily guarded Black Gate.
The pass is controlled by a fortress of the same name and guarded by Shelob. Frodo and Sam survive the spider’s attack, but Frodo is later captured by Orcs at the tower after Sam temporarily takes possession of the Ring.
Cirith Ungol is distinct from Cirith Gorgor, the pass containing the Black Gate, and from the Morgul Vale below Minas Morgul. The names refer to separate locations around Mordor’s approaches.