Mindfulness is the awareness of the mind’s activities and movements, and the effect of objects in the mind. It is a moment-by moment awareness of oneself and the experience of being a living breathing being. There is a “knowing” of phenomena that occurs in addition to the sense phenomena themselves. Knowing the mind’s state and activities is generally referred to as mindfulness.
Mindfulness is necessary to realize insight, because the meditator has to observe not only the contents and objects within the mind, but mind nature itself- there is an aspect of self-knowledge that is baked into the broad term “mindfulness”, and it seems like every meditation teacher has their own definition of mindfulness.
Any contemplative practice inherently develops mindfulness. Even in samatha practice, we put a high premium on the faculty of mind that “knows” what it is doing on a moment-by moment basis. We rely on noticing a distraction as a distraction – that noticing is mindfulness. Mindfulness is how insight experiences become breakthroughs of insight. The mind of a mindful meditator has the primary object in the center of their awareness and they have a sense of a broader picture and context for where the object fits in. Maintaining an awareness of the full landscape, while keeping the primary object in the center, that is mindfulness.
“ On whatever occasion the monk remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world, on that occasion his mindfulness is steady & without lapse. When his mindfulness is steady & without lapse, then mindfulness as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.
This indicates that “mindfulness” is a practice that can be applied without lapsing. We can always be aware of what our minds are doing, even when we aren’t specifically trying to meditate. A strong mindfulness practice will also be a more continuous practice- the mind is continually in a state of remembering and knowing itself in real time.