Text readability
Also said as Legibility, Contrast, HUD readability
How quickly and reliably someone can read what the interface is telling them while busy doing something else. For text it comes down to size, weight and contrast against whatever happens to be behind it.
Your interface will be read over every frame of your game, not over the flat grey it was designed on. Text that looks fine in a mockup disappears on snow, and a player mid-fight will not squint.
Say it
“All HUD text gets a two pixel dark outline, and anything longer than three words sits on a plate at 75% black. Check it against the brightest level we have.”
Where you’ve seen it
The Last of Us Part II has a High contrast display option, a separate way of drawing the game that mutes the scenery and gives allies, enemies and items strong, distinct colours.
Not to be confused with ui scale
Text readability is the whole question of how easy the words are to read. Scale is one answer: how big they are.