What is Tacit Knowledge?

Legal Definition
Tacit knowledge (as opposed to formal, codified or explicit knowledge) is the kind of knowledge that is difficult to transfer to another person by means of writing it down or verbalizing it. For example, that London is in the United Kingdom is a piece of explicit knowledge that can be written down, transmitted, and understood by a recipient. However, the ability to speak a language, knead dough, play a musical instrument, or design and use complex equipment requires all sorts of knowledge that is not always known explicitly, even by expert practitioners, and which is difficult or impossible to explicitly transfer to other users.
-- Wikipedia
Legal Definition
Unspoken, unwritten and hidden store of knowledge held by every human being that is based on experiences, emotions, intuitions, insights and observations.