adit

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An almost horizontal entrance to a mine.

from The Century Dictionary.

• noun An entrance or a passage; specifically, in mining, a nearly horizontal excavation, or drift (which see), specially used to conduct from the interior to the surface the water which either comes into the workings from above or is pumped up from below.
• noun Milit., a passage under ground by which miners approach the part they intend to sap. Wilhelm, Mil. Diet. Admission; access; approach.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun rare Admission; approach; access.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A horizontal or nearly horizontal passage from the surface into a mine as contrasted to a shaft which is a vertical entry passage. An adit may be used for ventilation, haulage, drainage, or other purposes.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a nearly horizontal passage from the surface into a mine