A meditation is a consciously induced nonphysical experience, and there are many kinds, only some of which may seek to fully venture into astral territories. Mantra or breath meditations are good for calming and cleansing the self, perhaps prior to an astral journey. Guided visualizations are like dreams that we initiate, full of coded messages which help to reveal and order one's own life and create desired effects. In a light trance like mediumship we can receive and relay messages from astral beings, while full-trance channeling is a stepping aside for an astral or higher being to use our bodies to say hello and gesticulate while we remain unconscious (possession is not an option - like hypnosis, no one can be made to do anything against their will). Shamanic journeying is pragmatic meditation, usually to the beat of a drum, which goes as far as necessary to retrieve healing information - a shaman may be doing visualization, mediumship, or full astral voyaging.
An astral meditation would be any willful practice that puts the body into a fully "parked" mode (safely mind-awake body-asleep), enabling the body's owner to then consciously exit with all senses intact, or at least soon to open (a transition of vertigo and darkness may precede the shift to astral eyes and ears, etc.) Thus we can understand that while astral voyaging is more dramatic and engaging than most
meditating, it is on the continuum of meditative experiences. Sleeping and death also enter astral realms, but since these are part of our unconcious cycles they are not generally considered meditative.