[kuh m-plee-shuh-nist]
  1. (in a video game) a player who attempts to complete every challenge and earn every achievement or trophy: I’m not really a completionist, so I skipped the side missions and focused on the main story quests.
  2. completist(def 1).

Origin of completionist

First recorded in 1970–75; completion + -ist Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018