"Adds"[]
Quite often, within PvE boss fights, Mobs that are not one of the main boss/bosses are referred to as Additional Enemies, which abbreviated becomes Adds. Adds quite often are often are not Champion mobs.
Trash Mobs vs. Adds[]
Trash mobs are low tier mobs/enemies that have no correlation to a boss fight, they are quite often located in between boss fights or boss rooms and often feature little to no mechanics. Gods From the Machine and Master Mode Operation Trash Mobs are an often an exception, featuring dangerous mechanics to challenge and slow the group down during Master Mode Timed Runs
Adds are within boss fights or encounters with varying degrees of mechanics and difficulty rating.
"Add Rush" Flashpoint/Ops Encounters[]
Add Rush encounters are fights in Operation or Flashpoint Instances that contain No Boss, instead consist of several Adds and a main objective. Often requires movement of the Ops group from one location to another or the complete opposite, a holdout against the adds until a condition is fulfilled.