Meta UI
Also said as Meta interface, Screen-space effects
Interface that stands for something in the story but is drawn on the screen rather than in the world: blood on the camera when you're hurt, a phone that fills the screen. It is in the story but not in the space.
It tells you something urgent without a single number, and it is hard to miss. It says little, though, and it covers the picture just as you need to see, so it needs a cap and an off switch.
Say it
“Show low health with a red vignette that starts at 40% and grows to the corners by 10%, and never let it cover more than a third of the picture.”
Where you’ve seen it
Call of Duty: Modern Warfare 2 shows damage as blood spreading over the screen, with no health bar. Grand Theft Auto IV has you take out a phone in the world, and then uses it through a flat screen overlay.
Not to be confused with hud
A health bar means nothing in the story, so it is non-diegetic. Blood on the screen stands for a wound, so it is meta.