adjective Possessing some quality in the third degree; especially having been subjected to the substitution of three atoms or radicals.
a tertiary alcohol, amine, or salt
adjective Of quills: growing on the innermost joint of a bird's wing; tertial.
noun Any item considered to be of third order.
noun A tertiary colour.
noun Something from the Tertiary Period (the former term for the geologic period from 65 million to 2.58 million years ago).
noun A tertiary feather; a tertial.
noun 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.
A gigaton-scale weapon would of necessity be a three-stage design (incorporating a fusion tertiary - likely with its own massive fission tamper - in addition to the fission primary and fission-jacketed fusion secondary), as a reasonably-sized primary would likely have difficulty imploding to ignition the enormous secondary that would be required to reach a one-gigaton yield with a two-stage thermonuclear.