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 and the Halo 3 Multiplayer map Isolation..

Trivia

 * The Porta will only open for the Master Chief and Covenant Elites, not Pure or Infection Form Flood. Because flood forms can't open portas, many players run past the waves of flood on higher difficulties.
 * The "Porta" was recently parodied in VG cats #240, "...gross"
 * There is also one Porta located on the Halo 3 Multiplayer map, Isolation.
 * Portas make a unique sound when they open or close.
 * Portas may or may not be the doors on High Charity, covered in Flood spore sacs.
 * Portas are constructed of 7 segments, another 7|7 reference.
 * Portas are similar to human heart's valve. Any significance of this is up to an individual's interpretation.