The 1913 edition of Webster's dictionary defines meditation as:
The act of meditating; close or continued thought; the turning or revolving of a subject in the mind; serious contemplation; reflection; musing.
and its modern-day descendent, the Merriam-Webster dictionary online, defines meditation as:
a discourse intended to express its author's reflections or to guide others in contemplation
These pair of definitions sum up the first sense of the word meditation as type of thinking. However, these days it is more common to think of meditation as something essentially different from thinking, or at least of a kind of thinking that is all-inclusive rather than discursive. The 1913 version gives a hint of this with its second definition (which it says is obsolete):
Thought; -- without regard to kind.
It is with this second meaning, or group of meanings, that I am interested, and below is a selection of definitions (culled from Google at random) of meditation in this second sense:
Meditation is the process of conscious, controlled focus of the mind which may take place when the thinking processes, both in pictures and in words, have been stopped.
Meditation is a set of attentional practices leading to an altered state or trait of consciousness characterized by expanded awareness, greater presence, and a more integrated sense of self.
Meditation is effortless concentration.
Meditation is to still the mind, focus it away from the everyday concerns of your talking self, and to listen inward.
Meditation is a way to evoke the relaxation response and at the same time a way to train and strengthen awareness; a method for centering and focusing the self; a way to halt constant verbal thinking and relax the bodymind; a technique for calming the central nervous system; a way to relieve stress, bolster self-esteem, reduce anxiety, and alleviate depression. ...But I would like to emphasize that meditation itself is, and always has been, a spiritual practice.
The goal of all forms of meditation is single-mindedness -- to let go of all distractions and focus on one object of attention or devotion.
Meditation is simply witnessing your mind without any involvement at all.
Meditation is to seek inner silence and losing the sense of separateness.
Meditation is a spiritual practice that bonds the mind, body and soul together.
Prayer is when you speak...Meditation is when you listen
Meditation is the intentional self-regulation of attention, in the service of self-inquiry, in the here and now.
Yang meditation is the concentrated focusing of the mind on something. The 'something' can have almost infinite variety. Common subjects of this type of meditation are: mantras, chakras, colors, shapes, prayers, and affirmations.
Yin meditation is the clearing of the mind of all thought, both pictures and words, and the holding of that mind in a focused and alert state.
Most of these definitions are good, some very good, and they all give a different nuance to the term.
Thanissaro Bhikkhu's definition:
Bring the mind into the present by focusing it on the breath, and then make a calm, mindful analysis of the processes of the mind as they present themselves directly to immediate awareness
Finally, a definition that I love, is this of Zen master Dogen (13th century). Although he said it of enlightenment, it can be applied equally to meditation:
Meditation is to be intimate with all things.