Zanshin is a Japanese term which is translated in English as remaining mind.
This was explained to me by one of my Sensei’s as being aware of your surroundings and ready, both mentally and physically, to immediately defend or attack after finishing a previous attack or defense.
Another Sensei explained it as the remaining film and drops of water on a glass after the water has just been spilled out. Even though most of the water has been spilled out, some still remains.