There is no clear definition of the word. Experiences can vary in so many ways.
It usually means that the effects of the psychedelic you´ve taken override your normal sensory perception, wich can lead to OBE´s, experiences of other worlds that feel very real, etc.
You´re flooded with psychedelic input until you will no longer stay afloat in normal reality, but 'go under' (often it also feels that way, especially at the beginning).
Vaporised DMT also acts quickly and because of this has an extra 'stunning' dimension to it.
But oral psychedelic´s like shrooms, mescaline, ayahuasca and ibogaine can also be this intense and overpowering, though less rapid.