Progress orbs are objects that appear in a Nephalem or Greater Rift upon defeating a Champion Monster pack or Rare Monster, which increase the progress bar of the rift by 1% when collected, hastening the summoning of the Rift Guardian. Three or four orbs are dropped normally, and a fifth drops when the Reaper seal of the Altar of Rites is unlocked in seasonal play. They make killing Elite Monsters a reliable tactic for clearing rifts, and consequently some strategies focus solely on elites.
The orbs appear in the game world and must be collected by moving close to them. Item bonuses and Paragon points improving the "Gold and Health Pickup radius" also apply to progress orbs, and the Reach seal of the Altar of Rites allows near-automatic collection (within 60 yards). Using Nemesis Bracers in Rifts leverages the progress orb mechanic by generating extra elite packs from Pylons - these packs drop the same amount of progress orbs as any other elite pack.
Regular progress orbs drop from the last Champion Monster of a pack, or from a Rare Monster even if its minions are not killed. However the extra orb from the Altar of Rites Reaper seal only drops when the full pack is killed including minions, much like the Bane of the Powerful Legendary Gem.
The progress orbs of the two types of rifts work the same, although Nephalem Rift orbs are golden orange, while Greater Rift orbs are purple. Both are slightly hazy looking and have skulls in the center, presumably representing the essence of the elite monster(s) killed.
Health Globes and progress orbs bear some resemblance as proximity-collected objects that appear upon defeating powerful monsters. However progress orbs have a much more strict and predictable pattern within the gameplay, and they have completely different benefits. Special powers or bonuses work only on one or the other and not both, with the exception of Pickup radius increases.
In Diablo Immortal, Progress orbs appear in the Challenge Rift and Elder Rift.