Possessing the ability to alter mood, anxiety, behaviour, cognitive processes, or mental tension; usually applied to pharmacologic agents.
Source: On-line Medical Dictionary, © 1997-98 Academic Medical Publishing & CancerWEB
There are many drugs that come under the banner psychedelics, and these all share the property that they affect the mind in a psychotropic manner, i.e they are psychoactive.
The main one you will head about will be LSD, but there are many others from the natural ones such as Cubensis mushrooms and Liberty Caps, which produce Psilocybin, to Mescaline containg plants, such as the cactus Peyote with Yage or Ayahuasca with natural DMT also being available, usualy taken with a MAO Inhibitor. Of course there is also a vast range of new "trendy" drugs out there, of which the vast majority will come under the Research Chemical banner.
I have personally had many wonderful occasions and experiences on psychoactive drugs, although I will admit I haven't had a chance to try as many of the natural varieties as I would like, but somehow I do not feel this is the place to go into it too deeply