Meditation is essentially the coordination of two states of consciousness with each other:
waking consciousness + subconscious = dream journey.
waking consciousness + deep sleep = silent meditation
waking consciousness + ecstasy = one-pointedness
waking consciousness + collective subconsciousness = invocation
There are also (for advanced practitioners) combinations of multiple consciousnesses, such as in mandala meditation.
There is also the opposite of coordination, i.e. the disruption of the integrity of consciousness as in sleepwalking, trauma, hypnosis, etc.
In meditation there are a variety of tools such as words (mantras), imaginations, postures (asanas, mudras and others), breathing rhythms, symbols and so on. There are also different dynamics such as associations, analogies, identifications and the like.
This results in a multitude of possible meditations and a colorful variety of possible experiences such as meeting one's own power animal, one's own soul or a deity.
Nevertheless, the main features of meditation can be traced back to a few simple basic structures.