Diegetic UI
Also said as In-world UI, Diegetic interface
Interface that exists inside the game's world, so the character could see it too: a meter on a suit, a map held in the hand. Fagerholt and Lorentzon's 2009 thesis Beyond the HUD names four kinds of game UI, sorted by two questions: is it in the story, and is it in the world's space?
It keeps your eyes on the world and makes the interface part of the fiction. The cost is clarity: when it is dark, or the action is fast, the information is harder to read and easier to miss.
Say it
“Move the health bar onto the character's back as a strip of lights, and add a limp under 30%. Keep a plain bar as an option for people who can't read that fast.”
Where you’ve seen it
Dead Space puts Isaac's health in a strip down the spine of his suit and his stasis energy beside it, and ammo counts on the weapon. In Metro 2033 the watch on your wrist shows how long your gas mask filter has left. Far Cry 2 has you look down at a map held in your hand.
Not to be confused with spatial ui
Both sit in the game's 3D space. Diegetic UI is something the character knows about. Spatial UI is there only for you.