noun

• (mythology, literature) A mytheme or trope in which the hero embarks on a journey to the underworld.
  "The ancient Greeks and the peoples of remote antiquity already knew of journeys of the soul, but these were often journeys to the infernal regions, descents into hell, catabases, with obstacles, such as encounters with various monsters, menaces of all sorts, the crossing of the bridge of the dead or the passage of mysterious rivers on foot or on horseback."
• (by extension, humorous) Any journey downwards or fall.
  "How did the man in the moon come down? The distance between the earth and moon is by no means inconsiderable, and other obstacles “too tedious to mention,” lie in the way of this famous Catabasis, which his lunar majesty is declared to have performed."
• (rare) The presence of downward (drainage or katabatic) winds.
  "Sasttrugi permit the identification of regions of developed katabasis and their direction, and therefore the general relief features in certain Antarctic regions."