Consciously directing your attention to alter your state of consciousness. Meditation has been shown to reduce stress, improve focus, alter the brain, and thus can be an aid in dealing with stress-related illnesses.
Meditation at its broadest sense is simply: focusing the mind.
Practitioners engage in meditation because it has a variety of benefits including: lowers blood pressure, calms stress responses, relieves pain, eases fatigue, elevates mood/helps relieve depression, ameliorates insomnia, and - over time - helping to enhance clarity and hones the ability to focus.
In other words, meditation - even for short periods - can help you quiet the chatter in your brain and focus your attention on something different. This acts as a brief mental vacation and allows you to return to the task at hand refreshed.
There are many forms of meditation: some developed over centuries of practice, but you may also find that you can tap into the meditative powers of simple, repetitive tasks of your own choosing. In the West, we tend to classify meditation into groups, though these classifications are a bit arbitrary and the actual practices will often have elements of more than one category.
The categories are:
focused or concentrative meditation
using an external source to focus on like a bell or mantra e.g. Qigong, Tibetan Buddhist, Zen Buddhist or Diamond Way, guided meditations
minding only your thoughts or the silence e.g. mindfulness meditation, Sahaja Yoga
automatic self-transcending meditation
reaching for higher levels of consciousness e.g. Vedic or Transcendental Meditation (arguably a form of focused meditation).
Different types of meditation have different benefits.
A study published in the journal Consciousness and Cognition found that concentrative meditation tends to activate beta and gamma brain wave activity when measured under EEG (beta waves are associated with normal wakefulness, while gamma waves are contentiously thought to be associated with our sense of self-awareness); open monitoring meditation is characterized by theta activity (deep relaxed or drowsy states); and self-transcending meditation is associated with alpha waves (wakeful relaxation).
Meditation styles can be woven with religion or completely secular. Choose a style that is in alignment with your spiritual beliefs and that matches your health and wellness goals.