As Shanna explained at the start of the Numenera Launch event at GenCon, “Iadace” is a word coined by the Order of Truth as a shibboleth.
A shibboleth is a word, sound, or custom that a person unfamiliar with its significance may not pronounce or perform correctly relative to those who are familiar with it. It is used to identify foreigners or those who do not belong to a particular class or group of people. It also refers to features of language, and particularly to a word or phrase whose pronunciation identifies a speaker as belonging to a particular group.
“Iadace” (pronounced yay-da-cee) essentially means, “I am one who understands, and I recognize that you are also one that understands.” In this context, understanding means an awareness of the larger world–that the strange environmental conditions like the weird geography and the iron wind come from manipulations in the prior worlds. That the numenera is not supernatural, but applications of science far beyond what anyone in the Ninth World can comprehend, although it can be comprehended. That through exploration and study, the numenera in all its forms can be mastered. That people can–even in this extraordinarily dangerous and bizarre world–take control of their destinies and build a true “ninth world” to rival the previous eight.
“Iadace” is used when a traveller comes upon a village or encounters a stranger on the road. Sometimes as a greeting, but also later in the conversation if you suddenly suspect the person that you’re speaking with to be an enlightened mind. It can also be used as a farewell.
Lastly, “Iadace” is used as a part of meetings of knowledgeable companions as a sign of shared understanding. Aeon Priests, for example, use it as a greeting among themselves all the time–it is practically a sacred word to them, as it embodies the whole essence of their outlook and goal.
So I say to all of you, “Iadace.”