The operational definition is Jon Kabat-Zinn's, who brought the practice into medicine through the MBSR programme in 1979: attention paid on purpose, in the present moment, non-judgementally.
What it is not: emptying the mind. The mind produces thoughts the way the heart beats, and the task is not to stop them but to notice the moment attention wandered and bring it back. The moment you notice you were lost is the practice, not its failure — this confusion is the main reason people quit in the first week.
Evidence is solid for a few specific applications and weaker for others. Mindfulness-based cognitive therapy (MBCT) is recommended by NICE for preventing relapse in recurrent depression, with significant reductions in people with three or more previous episodes. Results for anxiety and chronic pain are moderate and consistent.
It is not right for everyone, nor at every moment. For someone with unprocessed trauma, sustained attention to bodily sensation can trigger re-experiencing or dissociation. In those cases the work uses externally anchored practices and supervision, not long body-scan sessions.