Tacit knowledge is defined as the deeply personal, intuitive know-how that is difficult to articulate or write down. It is the "body knowledge" of a master craftsman, like the feel for a machine's subtle vibrations, which is impossible to formalize in a manual. 

In manufacture, a veteran worker's ability to sense a subtle change in a machine’s sound to prevent a breakdown is invaluable tacit knowledge that can't be put into a manual.

The process of sharing this tacit knowledge to create new ideas is the SECI model. It describes a dynamic spiral where tacit and explicit knowledge are converted into each other. To truly innovate, an organization must actively facilitate this knowledge creation process through direct human-to-human interaction and collaboration.