n. That part of a mine from which the mineral has been partially or wholly removed; the waste left in old workings.
the GNU version of the Collaborative International Dictionary of English
n. That part of a mine from which the mineral has been partially or wholly removed; the waste left in old workings; -- called also gob .
The Century Dictionary and Cyclopedia
n. A stack or cock, as of grain.
n. A rick of corn in the straw laid up in a barn.
n. 3. In coal-mining, a space from which coal has been worked away, and which is more or less filled up with refuse.
Word Usage
"But experts said that the preferred theory was that a collapse of the mine's roof in the "goaf" - the void left behind after coal is removed - released methane into the mine."