Heads-up display

A Head-up display, or HUD, is a translucent display that presents data while minimizing any obstruction to the user's view. Although they were initially developed for military aviation, HUDs are now used in commercial aircraft, automobiles, and other applications.

It should be noted, however, that the use of the term is technically inaccurate for the system used within the Halo universe, especially when describing it from an in-universe perspective. It is far more accurate to refer to the equipment as a Helmet Mounted Display.

Summary
The HUD is the main source for information in-game on Halo: Combat Evolved, Halo 2, Halo 3, Halo Reach, and Halo 4, though the appearance of the HUD has changed dramatically from its predecessors. The sensors augmented into the Spartan's body display the soldier's vital signs, which include a measure of their overall health in the form of bars. Note that this "health bar" has been removed from Halo 2 and Halo 3, with the reason being the insertion of automatic biofoam injectors into the Mark VI armor. In Halo 3, there a slight convex appearance to simulate looking through a real visor. Also, the MJOLNIR shields flare up in a grid-shaped pattern, to simulate the player is taking damage. Additionally, the HUD also reports the strength of the wearer's shields through the energy signatures it emanates. Weapons' heat and ammunition levels, as well as grenade type and count are also detected through sensors placed in the Spartan's gloves. In Halo 2 and 3, the Mark VI armor is able to register two weapons when the Spartan is dual-wielding.

Another notable feature on the HUD is that each type of weapon that the gloves detect has its own unique aiming reticule in relation to the type of ammo that it projects and it's statistics. Even heavy weapons have their own reticules. For example, the open circle reticule on the shotgun tells you that although the shot will land somewhere in the directed area, the distance and position are rather unpredictable. Or the Ghost's reticule tells you that the plasma shots will travel as far as the two dots show, but can drift off left or right a little. The reticule is also compatible with the scope of ranged weapons when installed. However, do remember that it is a reticule that is designed to aid the person in pinpointing where the shot will end up, while an aiming cursor shows exactly where the shot will end up.

The Motion Sensor is another important feature displayed on the HUD. It can sense and track movement in a 15 meter radius (Halo: CE), a 20 meter radius (Halo 2) and a 25 meter radius (Halo 3). Due to the insertion of IFF tags in UNSC soldiers, the radar can distinguish friend (yellow) from foe (red). Finally, the way point indicators are an on-screen directive that point out important objectives or locations in the Spartan's mission, and are extremely useful, measuring the distance the Spartan is from the objective.

A HUD is also featured on SPI Armor, MJOLNIR armor, ODST battle armor, Marine Body Armor, and even the Sangheili Combat Harness.

MJOLNIR Armor, SPI Armor, and Sangheili Armor
The standard MJOLNIR armor contains the following readouts:
 * (1) A meter displaying the relative Vitality of the user (absent from Halo 2, Halo 3, and Halo 4 ).
 * Grenade tracking. (only available on MJOLNIR GEN 2 armor)
 * (2) A gauge displaying Shield integrity status. (absent from ODST armor and SPI armor as it has no shields)
 * (3) A gauge tracking the quantity of ammunition available for weapons carried.
 * (4) A motion tracker. (absent from Halo 3: ODST)
 * (5) A counter tracking the number and type of Grenades carried.
 * (6) A targeting reticule which changes relative to the weapon currently in use.
 * (7) Acknowledgment lights from other members of the wearer's team.
 * (8) Maps and information uploaded over the wearer's TACCOM (seen only in Halo 3: ODST's VISR Database)--translated to a compass in Halo: Reach.
 * (9) Zoom Function.
 * Function for using a Fiber Optic Probe. (absent from gameplay)

ODST Armor and Marine Body Armor
The Heads-Up-Display used by UNSC Marine Corps and ODST's is similar to that used by Spartans, displaying weapons and grenades carried by the user and their ammunition, and projecting a targeting reticule to assist aim. However, it is notable for lacking several functions, such as the motion tracker and shield bar, which are unavailable for conventional UNSC personnel, though it does display health information. Additionally, ODST helmets integrate a VISR that improves the users vision in low light areas, also outlining objects in the immediate area - green outlines are "friendly" units, red outlines are "enemy combatants" and blue outlines are discarded or abandoned equipment and weaponry that can be used. The system can also track data and display tactical information, such as maps and waypoints.

Terminology
Although the games, novels and manuals all refer to a Head-Up Display (HUD), this is technically an incorrect description of the equipment featured in-universe. By definition, a Head-Up Display is a fixed unit where by you must look 'up' in order to see it. For example, the HUD in a fighter jet is mounted to the glareshield and cannot be moved. If the pilot looks to his left or right, he can no longer see the display.

The term HUD originally came from looking 'up' out of the cockpit as opposed to looking down at the gauges on the instrument panel. The correct in-universe term for the equipment in the Halo games is a Helmet Mounted Display (HUD), or HMD. A display mounted or projected onto a helmets visor, the result of which being that no matter where the wearer looks, the display will remain in front of his eyes. HMD's have been in military service as early as the late 1970s when the Soviet Union was developing the MiG-29 Fulcrum and are currently starting replacing HUD's in the newest generation of fighter jets, and the new Lockheed Martin F-35 Lightning II does not feature a HUD at all, instead completely relying upon the pilots HMD.

It is also possible that "Heads Up" originates from the idiomatic expression, meaning to "look out". Telling someone "Heads up!" means to alert them of an incoming threat, usually enemy fire, so one probable reason it was called "Heads Up Display" and not "Helmet Mounted Display" was to alert players about their current in-game status. Another reason is that HUD is used in almost all other games, but most of them being absent if the game were to be in real life.

Trivia

 * The Elites use either eye coverings to give them their HUD or a neural link like Marines use.
 * Custom games in Halo 3 allow you to change how far your radar reads (10 meters, 25 meters, 75 meters, or 150 meters). A downside to a large-range radar is that the red dots which represent enemies become smaller and harder to see.
 * It is unknown why the MJOLNIR HUD is able to place a targeting reticule on the viewer's screen for Covenant weapons and vehicles. While it is possible for ONI to have adapted reticles for captured Covenant weapons, however vehicles are much less likely to have been accounted for.
 * The HUD in Halo 2 is the only one in the trilogy that does not display any kind of information on the direction of the fire when the player is hit. Both Halo: Combat Evolved and Halo 3 utilize arrows that show the player where the shots came from; however, the arrows in Halo: Combat Evolved are much more general than the ones in Halo 3.
 * In Halo: Reach, when your shields are down, the word "Warning" will show up in the depleted shield indicator. Also, if your health is below yellow it will continuously flash.
 * It is unknown how the HUD changes from Halo 2 to Halo 3, as John possesses exactly the same armor in both games. While the changes are obviously for gameplay reasons, no in-universe explanation has been offered for them. It is possible that the armor has several HUD layouts available that can be changed according to the user's preferences. This also explains the radical differences between the HUD of Halo: Combat Evolved and that of Halo: Reach, as both games feature Mark V armor.
 * In the Halo: Reach campaign, the Heads Up Display gives the player a brief description of the weapon they are currently using by pressing the Back button.
 * In Halo 2, when you look up you see less of your weapon but when you look down you see more of it. This feature is missing in Halo 3.
 * In Halo 3 and Halo 3: ODST, the HUD seems to resemble the visor shape of the helmet.
 * In Halo: Reach, the Health and Shield bars look like the one from Halo: Combat Evolved, just with two of each bar facing each other.
 * In Halo 4, the HUD is fully 3-dimensional, and bobs and weaves with the player's movement. Additionally, portions of the player's helmet can be seen at the edges of the screen if he/she is in a first-person view.