Heads-up display

A Heads-Up Display, or HUD, is the name given to the screen viewed by the player of a first-person shooter game. In real life a HUD would be a person's view through some sort of visual-aiding device, often the cockpit window of an aircraft or a helmet mounted display.

In-game
In the Call of Duty series, the HUD is not present at all times, and appears only when the player is low on ammo, sometimes when switching weapons or picking up a new weapon, and some other times. In the "Hardcore" multiplayer mode the HUD is limited to nothing but a mini-map, and only when a UAV/Recon plane is called. They will also still see the names of teammates, the symbol showing they are carrying a bomb/flag, and the amount of flags captured.

Examples of things seen on a HUD include:
 * Names of teammates
 * Compass
 * Location of objective (in Call of Duty: Modern Warare 2)
 * Grenade indicator
 * Crosshairs (if enabled)
 * Reload/low ammo indicator (in Call of Duty: World at War and Call of Duty: Modern Warfare 2)
 * Equipped weapon
 * Current stance
 * Mission updates (New objectives, game saves, etc.)
 * Ammo counter (in Call of Duty through Call of Duty 3 it was just a simple numbered box, but in Call of Duty 4: Modern Warfare through Call of Duty: Modern Warfare 2 it was a visual representation of each remaining cartridge''
 * Weapon pickup icon/Use function
 * Health (health bar in Call of Duty, Call of Duty: United Offensive, and Call of Duty: Finest Hour. In Call of Duty 2 through Call of Duty: World at War the screen would slowly turn red as the player took damage, and get clearer as they stopped.  In Call of Duty: Modern Warfare 2, the screen would instead get covered in blood, and grow clearer as they stopped taking damage.  The screen will also get covered with dust if a grenade goes off nearby)
 * Game hints
 * Subtitles (SP), team chat (MP)
 * Callsigns (in Multiplayer)
 * Time left/passed (in Special Ops)
 * Enemies killed/remaining (in Special Ops)
 * Current round (in Nazi Zombies)