“Mindfulness is awareness that arises through paying attention, on purpose, in the present moment, and non-judgmentally.”
This is the oft-cited definition from Jon Kabat-Zinn, well-known author and founder of the Stress Reduction Clinic at the University of Massachusetts (who is careful to add that it is an operational definition and leaves out some aspects of mindfulness).
A similar definition comes from psychologist Scott Bishop, who describes mindfulness as a “nonelaborative, nonjudgmental, present-centered awareness in which each thought, feeling, sensation that arises ... is acknowledged and accepted as it is.”
Simple as it may sound, mindfulness transforms how we relate to events and experiences. It creates a more spacious way of being in the world that is less reactive and generally happier.