It is helpful to see knowledge in five distinct ways.
If we suppose that that the sum of possible knowledge in the Cosmos in a given moment is “common”. That we also suppose that an individual or group of people have a relationship with the “common”. We may consider that there exists with these two suppositions five types of knowledge.
This is knowledge we are unaware of, for instance electrons and black holes are “common”, but it was “hidden” knowledge to us until we invented microscopes and telescopes to reveal that knowledge.
If knowledge of a new way of communicating through photons is revealed by scientists, they may publish it in a journal, this knowledge may be known by all, but it is mere words on paper. This knowledge may be considered inactive knowledge, it only becomes “awake” if it is in use, thus it is “sleeping” knowledge.
With the destruction of the Druids people consider that the Druid knowledge is lost. We over time forget our own individual past. We have forgotten most of human history from more than 10,000 years ago, thus we are reduced to mere theory and speculation of what that human history may have been. All these are examples of knowledge once known, but now “forgotten” knowledge.
This is knowledge that is in use, or being actively used. I have knowledge on how to read and write English, or to tie my shoelaces, or not to look directly at the sun as it will burn out my eyes; these are all examples of Practical Wisdom. There may exist in a book knowledge on how to speak Spanish, but if it exists just in the book and not in my head, then it is “sleeping” knowledge.
This type of knowledge combines with either “sleeping” knowledge or Practical Wisdom. A barrier of some manner exists that separates the knowledge held by one group or individual from others. An example of this is the Freemasons, who pursue Practical Wisdom behind closed doors in secret. Another example is when the Druids transmitted their knowledge in secret from the Romans using Ogham. Another example is the nation state holding secrets that their citizens are denied access to. Note that “undisclosed” knowledge can take the form of “sleeping” knowledge or Practical Wisdom.