Originally, kokoro referred to the beat of the heart, which was considered to be the essential organ of life and the source of all activities. By extension, kokoro refers to all human activities affecting the outside world through intention, emotion, and intellect.
Kokoro, then, has three basic meanings: the heart and its functions; mind and its functions; and center, or essence.
The mind which is always on your side is not just your mind, it is universal mind, always the same, not different from another’s mind. It is Zen mind. It is big, big mind. This mind is whatever you see—this mind is at the same time everything. -Shunryu Suzuki Roshi