The practice whereby a person is intentionally aware of his or her thoughts and actions in the present moment, non-judgmentally. The 7th step of the Noble Eightfold Path.
Sati, translated as mindfulness or awareness is a spiritual or psychological faculty (indriya) that forms an essential part of Buddhist practice. It is one of the seven factors of enlightenment. "Correct" or "right" mindfulness (Pali: sammā-sati, Sanskrit samyak-smṛti) is the seventh element of the noble eightfold path.
Example via www.ramdass.org: Cultivating Mindfulness