Destruction

From TF2 Wiki

A destruction is a type of kill earned after destroying a building, either using a weapon or an electro sapper. A destruction is worth 1 point and can not be assisted unless the building has been sapped before by a spy in which case the spy gets the assist. You cannot get a destruction from destroying your own buildings. Destroying a building that is not yet completed still yields a destruction.

When a building is destroyed, the engineer responds with one of three response rules, alerting his nearby teammates.

"Sentry down!"