Mindfulness - Less a 'what', than a 'how'
Mindfulness, in reality, is perhaps best discussed as an adverb rather than a noun. Something active and in-process rather than something static. Of course, when we reflect, we naturally speak of things in a static nature, like a photographic snapshot of the past. For in our minds it has already happened. This tends to be the purview of the left-hemisphere of the brain.