Definitions of Mindfulness:
“Bringing one’s complete attention to the present experience on a moment-to-moment basis.” (Marlatt & Kristeller)
“Paying attention in a particular way: on purpose, in the present moment, and non-judgmentally” (Kabat-Zinn).
“The non-judgmental observation of the ongoing stream of internal and external stimuli as they arise.” (Baer)
“Awareness of present experience with acceptance.” (Germer, Segal, Fulton)
My own personal definition is: “Paying attention with openness, curiosity and flexibility.”