Porta

Were you looking for Portal, the Forerunner complex? The Porta is a sphincter-like orifice embedded in the floors and walls of Flood hives that act as the Flood analog to doorways. Portas were sighted in the Flood-infested High Charity.

Trivia

 * The Porta will only open for the Master Chief and Covenant Elites, not Pure or Infection Form Flood.
 * The "Porta" was recently parodied in VG cats #240, "...gross"
 * There is also one Porta located on the Halo 3 Multiplayer map, Isolation.