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

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "adit":
   access, admission, admittance, air lock, approach, channel,
   conduit, corridor, course, ditch, duct, egress, entrance,
   entranceway, entree, entry, entryway, exit, gangplank, gangway,
   hall, in, ingress, inlet, intake, means of access, opening,
   passage, passageway, trench, trough, troughing, troughway, tunnel,
   vestibule, way, way in