Atomic weapon and Fission bomb

Atomic weapon noun - A weapon of mass destruction whose explosive power derives from a nuclear reaction.

Fission bomb and atomic weapon are semantically related. In some cases you can replace phrase "Fission bomb" with "Atomic weapon", this phrases are similar.

Fission bomb noun - A nuclear weapon in which enormous energy is released by nuclear fission (splitting the nuclei of a heavy element like uranium 235 or plutonium 239).

Atomic weapon and fission bomb are semantically related. Sometimes you can use "Atomic weapon" instead a noun phrase "Fission bomb".