The Merriam Webster dictionary defines "mindfulness" as: "1. the quality or state of being mindful. 2 : the practice of maintaining a nonjudgmental state of heightened or complete awareness of one's thoughts, emotions, or experiences on a moment-to-moment basis; also : such a state of awareness."
Alternatively, another well recognized definition of mindfulness comes from Jon Kabat-Zinn who said, “mindfulness is paying attention, in a particular way, on purpose in the present moment without judgment.”
Check out these two videos on how mindfulness influences our brains and how it differs from its opposite "mindlessness." :)
https://www.youtube.com/watch?v=aNCB1MZDgQA
https://www.youtube.com/watch?v=1_h-Zn9pKAU