goaf

from The Century Dictionary.

• noun A stack or cock, as of grain.
• noun A rick of corn in the straw laid up in a barn.
• noun 3. In coal-mining, a space from which coal has been worked away, and which is more or less filled up with refuse.

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

• noun (Mining) That part of a mine from which the mineral has been partially or wholly removed; the waste left in old workings; -- called also gob .
• noun to remove the pillars of mineral matter previously left to support the roof, and replace them with props.

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

• noun mining That part of a mine from which the mineral has been partially or wholly removed; the waste left in old workings.