This is Shinzen Young giving a somewhat winding talk about world mysticism from a Buddhist perspective. I find his contrast (at 9:00 and following) between pseudo-mysticism – the experience of "weird stuff" (gods, ghosts, ancestors, acquisition of powers, etc.) – and Mysticism with a Capital M – which he defines as "touching the Formless Source" – to be helpful in understanding what people mean when using the term.
Most of this talk compares Eastern systems (Buddhist, Yogic, etc.), but at 40:30 he also addresses major Western traditions.