A Ferengi episode, or Annual Ferengi OutingTM is the term used for stories which revolve around Quark and other Ferengi. Largely used as comic relief to balance the dark nature of DS9, the Ferengi episodes have become notorious among fans due to the perceived trend that, starting with "The Nagus" (DS9 season 1), the quality of Ferengi episodes steadily declined, hitting rock-bottom with "Profit and Lace" (season 6). Although "Who Mourns for Morn?" has received some positive reactions from fans, "Profit and Lace" is considered the low point in the Ferengi episode chronology.
The Ferengi phenomenon originally started with TNG before being picked up by DS9. Originally introduced as a new threat for the TNG crew to face, the Ferengi were quickly relegated to comic relief status and paired with another phenomenon that has plagued fans, appearances by Lwaxana Troi, in "Ménage à Troi". However, the TNG crew have gone on record as stating that they felt the introduction of the Ferengi was a mistake, and they were later removed from the plot lines. It is worth noting that Quark did have a scene in Star Trek: Insurrection, which was removed. DS9 attempted to reinvent the Ferengi, although with mixed success.
After DS9, later incarnations gave their tip of the hat to the Ferengi phenomenon as well. In "False Profits" a pair of Ferengi accidentally find their way to the Delta Quadrant and manipulate a planet into thinking they are gods, while "Acquisition" fulfills ENT's legacy of destroying all things canon by introducing the Ferengi (who were unknown prior to the TNG era) in the 22nd century.