Seventh Column
From Halopedia, the Halo wiki
The Symbol
The symbol is a hollow ring with 7 rectangular points sticking out. The one on the top goes into the middle and is not connected to the rest of the ring.
What it Means
This has to do with Bungie's 7 steps to world domintation.
7 Steps to World Domination.
1.Start independent gaming software company. (Complete)
2.Dominate Mac platform: Launch assault on Windows platform. (Complete)
3.Announce killer gaming title. (Complete)
4.Acquire strangely addictive Chinese food company. (In progress)
5.Recover Ling Ling's head. (Complete)
6.Stage bloody coup of new parent company. (Complete)
7.Take over World. Shoot enemies into sun with giant slingshot. (In progress)
Bungie Puts the number 7 in many places in the game. If you want to read more "Seven Referances" click Here
Common places you see it
Many times if you're playing Halo 3 Xbox Live, many people have The symbol next to their name. This means they have signed into bungie.net. You can also see it around the game. In the Halo 2 level Headlong there is a statue that looks like the Symbol.