From the chapter "Aiming and Sustaining" of Jon Kabat-Zinn's book Coming to Our Senses:
This sustaining is known in Sanskrit as samadhi, that focused quality of mind that is one-pointed, concentrated, and if not utterly unwavering, it is at least relatively stable. Samadhi is developed and deepened as the normal agitated activity of our minds stills itself through the continued exercise of our ability to recognize when the mind has wandered off the agreed-upon object of attention, in this case the breath, and to bring it back over and over again, without judgment, reaction, or impatience. Simply aiming, sustaining, recognizing when the sustaining has evaporated, then re-aiming and again sustaining. Over and over and over and over again. Like the fins of a submarine or the keel on a sailboat, samadhi stabilizes and steadies the mind even in the face of its winds and waves, which gradually abate as they cease being fed by our inattention and our veritable addiction to their presence and content. With the mind relatively steady and unwavering, any object we hold in awareness becomes more vivid, is apprehended with greater clarity.
^z - 2009-02-13