Porta

Were you looking for Portal, the Forerunner complex? A Porta is a sphincter-like orifice embedded into the floors and walls of Flood Hives that acts as the Flood analog to doorways. Portas were sighted in the Flood-infested High Charity and in the Halo 3 multi-player map Isolation. There are also 7|seven portas on the level Cortana.

Trivia

 * Portas may or may not be the doors on High Charity, covered in Flood spore sacs.
 * Portas have at least two 7 references in them, one being that Portas are made up of 7 segments, and that there are 7 Portas on the level Cortana.
 * The name, "porta", probably comes from the Greek word porta (πόρτα), which means door.
 * On the Halo 3 multiplayer map Isolation there is one porta. Sliding down a hole on the top part of the map will lead you to slide down a tube. A porta will open, letting you into the Flood-infested cave.