Definition of "tertiary"
tertiary
/ˈtɜː.ʃi.ə.ɹiː/
noun
plural tertiaries
(military) A large stage in some extremely powerful thermonuclear weapons (resembling a greatly-enlarged secondary) which is compressed by the explosion of the secondary until ignition of nuclear fusion takes place, in much the same manner as the secondary is imploded by the primary, and which can allow for the attainment of yields of many tens or even hundreds of megatons, and likely even greater; not used in modern weapons due to a greater focus on the accurate use of sub-megaton weapons, the tremendous size of weapons incorporating a tertiary, and the lack of targets whose destruction would necessitate the use of a three-stage weapon.