HUD
Also said as Heads-up display, Non-diegetic UI, Overlay
The information laid over the picture of the game: health, ammo, score, map. The name comes from fighter planes, whose pilots read figures on the glass without looking down. Elements that exist only for the player, outside the world, are also called non-diegetic.
A HUD is the clearest way to tell a player anything, and each piece of it takes eyes off the world. Too little and players feel blind; too much and they play the corners of the screen.
Say it
“Cut the HUD to health and ammo. Fade both out after three seconds at full, and bring them back the moment either changes.”
Where you’ve seen it
Super Mario Bros. keeps score, coins, world and time in a single row across the top. Skyrim hides its health, magicka and stamina bars when they are full and brings them back when they are needed, while its compass stays. Journey has no HUD at all.
Not to be confused with ui
UI is the whole interface, menus and all. The HUD is the part on screen while you play.