About 'Siyee'
The word is taken from the Trudi Canavan fantasy series 'Age of the Five' and in-canon (slight spoilers ahead!) describes a species with formerly-human ancestors changed by a 'goddess' (magician with a god complex more like) to have wings and be smaller to be able to fly- which at some point worked, while also resulting in many deaths and cripples in failed attempts as well as disabled children in future generations.
The term 'siyee' or 'a siyee species' used by Central is taken from this species- either from the novel or if that story actually is part of the 'verse, I haven't quite decided yet- not first intentionally as an official term but rather as a comparison that at some point is used so often and with such a clear meaning that it is adopted into 'official' Central dictionary.
A siyee species is a humanoid or formerly humanoid species deviating from 'regular' humans in ways that have been or can be passed on to generations after the initial change took place. Siyee are generally changed by outside influence, not naturally occurring evolution. Whether voluntary subjects or self-experimentation counts- whether the term is therefore about an unnaturally changed species or about a mutilated people- is up for debate.
In later years, the term is also applied to non-humanoid aliens who have undergone similar changes, usually in addition with the original species, so e.g. 'the xy are an [alien species]-siyee race'.
Whether people afflicted by a disease that changes their DNA- for example lycantrophy (werewolves) count as siyee is also up for debate. In most cases, victims of illnesses who can be passed on to the next generation are called siyee if the individual person identifies with said term.
'Second Generation' Siyee are people who have not undergone changes themselves, with the exception of people who have undergone the changes as infants or so long ago that they cannot remember/have lived longer as siyee than not. Other than first generation siyee who tend to find their new bodies alien and, if given the choice, want to change back to their initial form (of course, exceptions prove the norm here too), second generation siyee grew up in their siyee bodies, tend to want to stay the way they are and identify with their species and culture.
(Humanoid) Siyee species in the l-verse, (in alphabetical order) including but not limited to:
Bajorans
Dwarves
Elves (Several Types)
Hobbits
Klingons
Shifters
[Siyee]
Vulcans
For other species, their origin is unclear- they could have therefore theoretically been created via natural evolution. This includes several types of vampires, demons, faries and giants.
