adit

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An almost horizontal entrance to a mine.

Century Dictionary and Cyclopedia

1. n. 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. The word tunnel is in general use in the United States, and especially in the western mining regions, for adit; but the former properly signifies an excavation open at both ends, such as is used in railroads. When there are two or more adits, the lowest is called the deep adit. Adits are occasionally several miles in length. The so-called Sutro tunnel, draining the Comstock lode at Virginia City, Nevada, is the most extensive work of this kind yet constructed in the United States. It is about 20,000 feet in length, and intersects the lode at a depth of about 2000 feet. Also called adit-level. See cut under level.
2. n. Milit., a passage under ground by which miners approach the part they intend to sap. Wilhelm, Mil. Diet. Admission; access; approach.

Wiktionary

1. n. 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.

GNU Webster's 1913

1. n. 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. n. Admission; approach; access.

WordNet 3.0

1. n. a nearly horizontal passage from the surface into a mine