It is short for Head-up display and is prominent in novels that feature LitRPG or game like elements.
May also be termed as System HUD. It will most commonly display elements such as a Health, Mana, Strength, Dexterity, Intelligence, Wisdom, Charisma, Skills, Levels, Name and Race and quantify them. May also offer a short description of each when prompted by a competent User through verbal, physical or mental means.
Usually in a blue color and notifies the User of any new changes such as creatures killed, level ups or special achievements.
Joe-"Dang this System HUD sucks, and I could have designed a better one! One that quantified my health and mana would have been great."