The Collaborative International Dictionary of English v.0.48:

adit \ad"it\ ([a^]d"[i^]t), n. [L. aditus, fr. adire, aditum, 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.
      [1913 Webster]

   2. Admission; approach; access. [R.]
      [1913 Webster]

            Yourself and yours shall have
            Free adit.                            --Tennyson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

adit
    n 1: a nearly horizontal passage from the surface into a mine