Notte’s word – borrowed from the concept of precious gems being uniquely flawed – for the one-off magical abilities his children sometimes manifest.
The existence of these gifts is kept as a very large secret for two reasons. One, while all Night Children have that potential, some never discover that skill, and some can take thousands of years to manifest it. Two, should the Mythos realize that Night Child blood can turn humans from non-magic users into magic users, they would collectively lose their minds. Therefore, Notte almost never brings it up until the Night Child in question has begun to show their unique talent.
As a side note, Raven refuses to use that word. She believes only a perfect gem has worth, and finds this “flawed, unique beauty” concept disgusting.