Flag

A flag is a piece of cloth, often flown from a flagpole and emblazoned with a design used to represent a faction or nation.

Canon
The flag of the UNSC consists of a blue field with stars and Earth at the corner. Human rebel forces possess several flags throughout the various insurrection movements, including insignia such as family crests, bloodied dragons, scorched crossed swords, and a fist.

Multiplayer
In most of the Capture the Flag gametypes, the flag is an item that players must retrieve from the opposing team's base and return it to the players' base in order to score points. In some gametypes such as Assault, players' have to transport the flag towards the opposing team's base in order to score points. Similar to Skulls, the flag temporary replaces the previous weapon the player was using when carried. The flag can't be holstered, but can be used as a very efficient melee weapon. Depending on the rules of the variant, the flag can deal much more damage than a normal melee would. In the default settings for CTF in Halo 3, a flag melee can cause an instant kill.

Production Notes
The flag's design in Halo: Combat Evolved was very plain, with very little ripple animation or pole design. It is topped by a skull with a crescent moon shape below it. Halo 2 saw a number of visual upgrades to the flag, but the overall design and shape of the flag is still similar to its predecessor. The flag in Halo 3 is entirely different than that of the previous two games. The Halo 3 flag is more of a banner than a flag. The flag is held onto the pole by one parallel bar, perpendicular to the pole, and flaps little between them. The emblems have changed; now they bear a symbol of the UNSC, and the only defining characteristics are their colors. In Halo: Reach it has changed back to a flag. It now retains a silver skull on top and sports the logo of Noble Team, along with a slight shine to the lining.

Trivia

 * On Bungie.net, the description of the multiplayer flag states that "It's right next to the headlight fluid." This is a reference to Red vs Blue, in which Donut is sent on a fool's errand to get headlight fluid and inadvertently captures the Blue flag.
 * In Halo 3, if you stick an opponent with a Plasma or Spike Grenade, then pick up a flag before the grenade explodes, the kill will count as a Flag Kill, this does not work with the Oddball, however.
 * As part of their training, the SPARTAN-II trainees often participated in Capture the Flag exercises against companies of UNSC Marines.