soffit

noun
  1. 1

    The visible underside of an arch, balcony, beam, cornice, staircase, vault or any other architectural element.

  2. 2

    (pipe technology) The top point of the inside open section of a pipe or box conduit.

    โ€œThe elevation of the pipe soffit is 10.4 meters above mean sea level.โ€

Source: https://en.wiktionary.org/wiki/soffit