The distinction between these two types of weapon is blurred by the fact that they are combined in nearly all complex modern weapons: a smaller fission bomb is first used to reach the necessary conditions of high temperature and pressure which are required for fusion. Fusion elements may also be present in the core of fission devices as well as they generate additional neutrons which increases the efficiency (known as "boosting") of the fission reaction. Additionally, most fusion weapons derive a substantial portion of their energy (often around half of the total yield) from a final stage of fissioning which is enabled by the fusion reactions. Since the distinguishing feature of both fission and fusion weapons is that they release energy from transformations of the atomic nucleus, the best general term for all types of these explosive devices is nuclear weapon.