Yoga Sutra 1:2--Yogah citta vrtti nirodhah--
"Yoga is the suspension of the fluctuations of the mind." -G. Maehle translation
"Yoga is the cessation of movements in the consciousness." -B.K.S. Iyengar translation
Mr. Iyengar describes yoga in his commentary on this sutra as "both the means and the end". So, it is yoga when I step onto the mat and work to pay attention to breath, to driste, and to bandha. Occasionally, when there is a brief quiet pause between thoughts, that is yoga too. One is yoga as the means, the other is yoga as the destination.
Maehle translates "vrtti" as fluctuations in the mind and Iyengar translates "vrtti" as movements in the consciousness. Fluctuations and movements sound so innocuous to me. They sound like something that should be easy to manage. Another translation (which I unfortunately did not make a note of) translates "nirodhah" as "no storms". I like that. When I think of all that goes through my mind during the day it feels more like storms than mere movements...hence my love of and need for this yoga practice. :)