Fission bomb and Thermonuclear bomb

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).

Thermonuclear bomb and fission bomb are semantically related in nuclear bomb topic. In some cases you can use "Thermonuclear bomb" instead a noun phrase "Fission bomb", when it comes to topics like atomic bomb.

Thermonuclear bomb noun - A nuclear weapon that releases atomic energy by union of light (hydrogen) nuclei at high temperatures to form helium.

Fission bomb and thermonuclear bomb are semantically related in nuclear bomb topic. You can use "Fission bomb" instead a noun phrase "Thermonuclear bomb", if it concerns topics such as atomic bomb.