JAM(capitalized) was the code-name for a weapon used by the Gudersnipe Foundation during the Kamian Succession Wars. Its use was key to holding the line mid-point during the war and vital to finally bringing the conflict to a close.
The Succession Wars saw the wide-spread use of Terchenski's Particle by both sides, effectively blinding active sensors and severely limiting many types of passive scans. Combatants were forced, in many cases, into very close-quarters combat, in which the Kamians had a distinct advantage. With the primary threat being capitol ships (effectively immune to stunt fighters and the sorts of fast-attack craft that could survive in close-proximity), the Foundation began searching for a way to gain back a superior tactical edge. Foundation ships had much better sensors, but lacked the ability to see well through their own Terchenski fields, let alone the second layer of fields generated by enemy ships.
JAM reffered both to a substance and to its use. The substance was basically a special high-intensity radio-active isotope mixed with a high-heat synthetic polymer. This slime (which did resemble preserves, but was very bad on toast) was wrapped around a chemical explosive and used as a warhead on the fastest generation of SIM missiles available. These missiles, which caused no damage to the targets, were used to "paint" enemy capitol ships. The isotope, which stuck to the hull, made it easily detectable from great distance.
JAM-equiped sims were carried as external armaments on both Harpies and Raptors (the Hawk was phased out by the time JAM was introduced). Since the JAM missiles still functioned effectively as SIMs, each fighter or bomber would launch equipped with one. That way, whoever got close enough would still have the ability to "JAM" the enemy.
Once a target had been painted, it became much more visible to passive sensors, and made targeting easier.