an underground passageway, esp to make or force a way (through or under something), any passage or channel through or under something.
[Middle English tonel, tubular net, from Old French tonnelle, diminutive of tonne, tun, possibly of Celtic origin.]
Noun 1. tunnel tunnel - a passageway through or under something, usually underground;
1. To make a tunnel through or under.
2. To produce, shape, or dig in the form of a tunnel.
3. A depression hollowed out of solid matter
4. A long vertical passage sunk into the earth, as for a mine or tunnel
5. A hole made by an animal, usually for shelter