tertiary

1. Any item considered to be of third order.
2. A tertiary colour.
3. Something from the Tertiary Period (the former term for the geologic period from 65 million to 2.58 million years ago).
4. A tertiary feather; a tertial.
5. 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.
6. A member of a Roman Catholic third order; a layperson who participates in activities similar to those engaged in by men and women who take religious vows (respectively the first and second orders), and who may wear some elements of an order's habit such as a scapular.

Plural tertiaries