Fusion bomb

From Memory Alpha, the free Star Trek reference.

Fusion bombs were among the weapons employed by Vendikar (and, presumably, Eminiar VII) in their war. According to Mea 3, they were "materialized by the enemy over their targets". In fact, they existed nowhere outside the complex computer models that described the war. Nevertheless, each successful "hit" resulted in massive casualties -- and these individuals had twenty-four hours to report for disintegration. (TOS: "A Taste of Armageddon")

It is possible that these fusion bombs were actually a type of thermonuclear weapon such as the hydrogen bomb, but this is pure speculation.