by Thanissaro Bhikkhu
What does it mean to be mindful of the breath? Something very simple: to keep the breath in mind. Keep remembering the breath each time you breathe in, each time you breathe out. The British scholar who coined the term “mindfulness” to translate the Pali word sati was probably influenced by the Anglican prayer to be ever mindful of the needs of others—in other words, to always keep their needs in mind. But even though the word “mindful” was probably drawn from a Christian context, the Buddha himself defined sati as the ability to remember, illustrating its function in meditation practice with the four satipatthanas, or establishings of mindfulness.
“And what is the faculty of sati? There is the case where a monk, a disciple of the noble ones, is mindful, highly meticulous, remembering & able to call to mind even things that were done & said long ago. (And here begins the satipatthana formula:) He remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world. He remains focused on feelings in & of themselves... the mind in & of itself... mental qualities in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world.”
— SN 48.10