The term "most fissionable" refers to a substance's ability to undergo nuclear fission. Some synonyms for this term include "highly fissile," "easily fissionable," and "efficiently fissionable." Additionally, the phrase "fissile material" is often used to describe substances that can undergo nuclear fission, such as uranium-235 and plutonium-239. It's important to note that these materials can also be described as "nuclear fuels" or "reactor fuels," as they are commonly used in nuclear reactors to produce energy. When selecting materials for nuclear reactors or weapons, their fissionable properties are a critical factor to consider.