A shaft used wholly or mainly for ventilating mines, for bringing fresh air to places where men are working, or for exhausting used air.

This term is one of the over used ones. There is a lot of effort in digging a mine. You don't do it if you don't have to. The airshaft is usually an abandoned shaft which has been intersected during the normal course of following the common denominator (usually a quartz vein) or driven a small distance to or along a vein to a known abandoned or disused shaft. An intersection provides ventilation. Two entrances to the surface provide a natural airflow, hence the term airshaft.