Adonalsium has to do with the Cosmere, which is the word Realmatic Philosophers use to refer to the greater universe of the Shardworlds. It’s hard to separate religion, magic, science, and society in most of these worlds.
The word Adonalsium (or some misspelling of it) appears in only a couple of the books. Other clues and links between the books can be found as well. (Some people on my forums have spotted some of them. Others have gone unspotted so far.)
So who are the Realmatic Philosophers?
Realmatic philosophers are people in the Cosmere who have theorized on the relationships between the Physical, Cognitive, and Spiritual realms. At least their philosophies will play roles in future books.