Burnout

Burnout is created by casting any spell, which increases all spell mana costs by different amounts which vary in level and the spell being cast, offensive is usually the most costly to cast depending on modifications and type of spell it is e.g. lightning projectile.

Burnout decreases naturally at around 2 per second and when burnout is max you will not earn any XP from using the spell(s) thus slowing down the magic level advancement.

Compendium Entry

Burnout is your level of strain when casting spells. The higher your Burnout, the more mana each spell will cost. Casting more powerful spells will cause more Burnout. It will decrease over time. Gaining Magic Levels will give you a higher Burnout cap, allowing you to cast more spells before becoming completely fatigued. Your Burnout level is displayed as a red bar on the HUD. You can use "/amuicfg" to toggle an option to show the actual numeric values of your burnout.