Psyche-delic, something that reveals the self, the mind, the soul.
For me psychedelic is everything that reveals something about myself or enables me to understand something about the world around me and my place inside it.
Certain substances can certainly be a big psychedelic trigger but do not necessarily need to be. A yogi may use cannabis as a strong psychedelic plant, while a common stoner might just use it to laugh about the newest episode of How I met your Mother. Of cause the psychedelic potential differs with the substance.
Music, literature and art can definitely be psychedelic. Living things like trees, birds, fish, insects... all kinds of plants, animals and other liveforms can be severly psychedelic, especially in combination with certain substances.

I have a seawater reef aquarium and I can certainly say that marine animals are pretty psychedelic. :lol:
Similarly the ocean and other waters, mountains, the vulcanic fields of island or other landscapes, conversations and sciences like biology, physics, chemistry... Although they are "just" more suffisticated observations of some of the things already mentioned.
All in all there are many things that can be called psychedelic, but none of them necessarily has to be. Maybe the most psychedelic thing is the mind itself, if you allow it to observe itself and let it run free through awareness, meditation, budo, sports.. whatever works for the person in question. Of course all of these things can be supported by certain substances. :roll: