Corpse running

From WoWWiki

Nothing is certain except death and repair costs.^{TCGFoO 102}

Corpse running means to run your character's ghost or wisp to the site of your fallen body to resurrect there.

It was often used to run as fast as possible back to an ongoing encounter to be able to help again after dying. However, Blizzard decided that this was not an intended option to ease raid encounters so they introduced an instance block that prevents you from entering the instance in ghost form or alive while your raid is fighting a boss inside.

Since the same measures cannot be used for non-instanced raid encounters like Azuregos, most world bosses have special abilities to prevent corpse running. For example, if you die during an Azuregos fight you will get a debuff called Mark of Frost that will freeze you in place if you get within 100 yds of Azuregos.

There are however a few encounters that allow you to corpse run, e.g. the fight against Bloodlord Mandokir in Zul'Gurub where special Spirit Healers allow you to resurrect within the instance and without any resurrection sickness.