An explosive device was any tool or weapon that used an exothermic reaction created by an explosive material to provide an explosion of energy.
Jean-Luc Picard holding a Koinonian explosive device
The Koinonians became embroiled in a war that lasted for several generations that finally destroyed the entire race. Centuries after their extinction, explosive devices using subspace proximity detonators that had been buried remained hidden in underground caverns and tunnels on the Koinonian homeworld. In 2366, one of these explosive devices was accidentally tripped by Lieutenant Marla Aster of the USS Enterprise-D, instantly killing her. (TNG: "The Bonding")
As of 2401, Koinonian explosive devices were stored on Daystrom Station. (PIC: "The Bounty")