It's a broad term and can mean different things in different contexts. Even in the "religious or spiritual sense" it varies.
In the mystical sense, I take it to refer to a type of persistent seizure -- a status epilepticus in medical speak -- often associated with the left temporal region of the brain (though I suspect a global involvement in some of the 'higher' states). Note I'm not referring to the tonic-clonic type seizure people usually picture when they hear 'seizure'. Medically, seizures are any "short circuit" of the brain and need not present as loss of motor control.
In a mystical enlightenment, the experience is one of subject-object unity and ego-expansion. The degree of this mind expansion varies. here seem to be several stages.
It begins with an ego expansion. It's hard to explain, but it's an expansion of awareness and of self; an experience of expanded self, able to stand back and grok a larger picture, as if you'd previously seen only a few pixels and now see a whole landscape, like a flatworm suddenly gaining the insight of an Einstein or Huxley. There's no way he could ever communicate the experience to his fellow flatworms, as there's no way an enlightened human can communicate the experience to his fellow humans.
There is a sensory expansion. Instead of a three dimensional world and arrow of time, one experiences a world of six, eight, ten dimensions; a quantum or string reality; Plato's cave infinitely expanded.
Totally ineffable -- you have to have been there.
As the seizure extends, there's an increasing inability to distinguish self from non-self. Everything begins to be experienced as self. One becomes unable to distinguish oneself from a teacup; fr another person -- or a planet. One becomes the planet. One becomes the entire universe.
It's not a psychosis or hallucination. It's a real merger with the universe, like a drop of water merges with the ocean.
The experience is uncanny. Focus on your sleeve and your awareness flies into the weave. You can go as deep as you will, you can see the threads, you can see the anatomy of the fibers, the molecular structure, the atoms themselves. You experience the weaving, the mind of the weaver and the designer, you become them, you live the life of the original discoverer of weaving. There's no end to it.
Expand your focus and your consciousness balloons out into the universe. Space, time and distance merge and are transcended. You experience past, present and future as a Gestalt; as a massive now. You experience all places as an integrated here. Consciousness, too merges into a single, omniscient superconsciousness.
