From AMS Glossary
A form of artificial lightning discharge initiated with a rocket trailing wire that may or may not be connected to the ground.
The first phase of the discharge is a unidirectional leader starting from the tip of the wire. When the low end of the wire is not connected to ground, bidirectional leader development occurs from both ends of the wire, similar to lightning initiation from aircraft. In the case of negative space charge overhead (usual summer thunderstorm condition), a triggered lightning may only be a positive leader or may become a sequence of dart leader–return stroke processes following the initial positive leader. The latter is analogous to the subsequent return stroke process in a negative cloud-to-ground flash with the initial positive leader being analogous to the first return stroke. In the case of positive space charge overhead (usual winter storm condition), the triggered lightning is a single negative leader.