Some Christians are a bit nervous about meditation because it doesn't quite fit their idea of what prayer is . Most Christians trying to maintain a regular prayer life, sooner or later come up with some pattern of prayer or other which involves talking(even if that talking is carried on within the confines of their own skull); for example the well used ACT pattern. In this, we go through a cycle of Adoration, Confession and Thanksgiving - telling God how great he is, telling him what we've done wrong lately and thanking him for whatever it is that has come our way lately: all stuff you might have thought he knew already. Usually we also ask for things to be done for us or for others, and again, presume upon the foreknowledge of the one who created our desires.
Down through the years many Christians have thought, 'surely there's got to be more to prayer than that', and in various ways and at various times, have begun and continued a great exploration of the human psyche which constitutes the huge mass of knowledge loosely called Christian mysticism. Within this tradition, meditation has been practiced for many many centuries. The desert fathers used to teach by giving their disciples a Word: a personal phrase for them to ponder repeatedly for years; in other words, a mantra. Meister Eckhart discovered and taught a discipline very like Mindfulness meditation. The anonymous medieval book of instruction The Cloud Of Unknowing assumed a mantra based meditation. From these medieval classics to the modern proponents of Christian Meditation such as Anthony De Mello and John Main the tradition has been strong, but also it has been regarded with suspicion by many Christians precisely because it is devoid of words and we Christians are usually very very fond of words - especially our own. How can it be prayer if you don't say anything? Or think anything? Or feel anything?
For those who are a bit nervous about it, there's a number of ways in which meditation is prayer in the usually accepted sense of the word as verbal communication with God. The use of a mantra can be a prayer: a repeated petition or act of worship. For many, a time of meditation is often ended by a brief period of intercession, in which the heightened consciousness and concentration of the meditation is brought to bear on some person or issue or other. For myself, I have certainly noticed that reading the Bible or the New Zealand Liturgy immediately after meditating invests familiar words with whole new depths of subtlety and meaning. But the way in which meditation is prayer is a bit more complicated.
Alan Firth has written a lovely narrative poem called The Gardener in which he tells the story of a grower of prize vegetables who wishes to communicate with the slugs who live in a wasteland beside his garden. Faced with the impossibility of communication across such a divide of perception and intelligence, he magically becomes a slug himself in order to talk to them. It is, of course a metaphor of the incarnation. It's a clever piece, both artistically and theologically, and one of its basic presuppositions is the enormous distance between God and humankind in terms of intelligence and consciousness. How could we possibly conceive of the mind which conceived our minds? When we talk to God, as we do in prayer - our feeble intellects making contact with The Old Wise One - it is slugs talking to the gardener. Our incoherent spur of the moment Eek! Save Me God! prayers or our most elaborate liturgies are all slugtalk. We are creatures trying to converse with the uncreated and we are working with all the limited resources of our creatureliness. The automatic patterns which govern almost all of our lives and our thinking don't leave off when we start thinking of or conversing with the Almighty. The language we use, the sense of our own self we bring to the conversation and the image we have formed of God are all part of the great complex of unconscious patterns which I spoke of a few days ago. All our language about and to God is limited. But when we manage to be still enough to leave the patterns behind us, we find ourselves able to break free of those limits.
Meditation is not in itself a religious activity; as a comment on an earlier post pointed out, an atheist could meditate without doing violence to his world view, but it does bring us closer to a perception of what IS - the universe freed of the preconceptions through which we normally view it. It brings us closer therefore to the God who defined himself as I AM. It might not look it on first glance but the prayer of utter silence is the purest prayer there is.