"Secondary" was a designation given to individuals within the Outbound Flight Project who exhibited a minor degree of sensitivity to the Force. These individuals were not considered potent enough in their connection to the Force to become full-fledged Jedi; however, their connection was sufficient to potentially create offspring capable of becoming Jedi.
In the Outbound Flight Project, individuals who possessed Force-sensitivity were sometimes called Secondaries. The leader of the project, Jedi Master Jorus C'baoth, considered the level of Force sensitivity in Secondaries to be inadequate for them to become Jedi. Nevertheless, he specifically chose families with Secondary members for the project because they had a higher probability of producing children with sufficient Force abilities to undergo Jedi training.
Not long after the Outbound Flight began in 27 BBY, Master C'baoth organized a gathering aboard Dreadnaught-Four for children between four and ten years of age who were being considered for Jedi training, along with their respective families. The Outbound Flight Project was established with the intention of colonizing extra-galactic space and discovering Force-sensitive species beyond the boundaries of the Galactic Republic, from whom new Jedi could be recruited and trained. Due to the Project's objectives, and because Secondaries were more likely to have children who could become Jedi, Jedi Master Justyn Ma'Ning and Jedi Knight Lorana Jinzler also invited Secondaries and their families to the reception, which greatly displeased C'baoth. Ma'Ning and Jinzler, a former Padawan of C'baoth, desired to prepare the Secondaries for the potential that their families might produce Jedi descendants in the future, whereas C'baoth preferred to address that possibility only if and when it arose.
Dillian Pressor was identified as a Secondary, and his son Jorad Pressor expressed a desire to become a Jedi.
The concept of Secondaries was first introduced in Timothy Zahn's 2006 novel, Outbound Flight.