Source : Webster's Revised Unabridged Dictionary (1913)

Adit \Ad"it\, n. [L. aditus, fr. adire, ?aitum, to go to; ad +
   ire to go.]
   1. An entrance or passage. Specifically: The nearly
      horizontal opening by which a mine is entered, or by which
      water and ores are carried away; -- called also {drift}
      and {tunnel}.

   2. Admission; approach; access. [R.]

            Yourself and yours shall have Free adit. --Tennyson.

Source : WordNet®

adit
     n : a nearly horizontal passage from the surface into a mine