A Psychonaut is an individual who uses Altered States of Consciousness (ASC) to explore the structure and content of the human psyche. Psychonauts often make use of entheogens to intentionally induce altered states; however, "accidental" exploration is possible as for example during the course of a brain stroke (Sharp, 2008), a Near Death Experience, or after extended periods of stress.
Sharp, Michael (2008). The "Childlike" Right Brain. Personal Blog.