In one blink of your eyes,
You have missed seeing."
A Hindu story tells of a fish who asked of another fish: "I have always heard about the sea, but what is it? Where is it?"
The other fish replied: "You live, move and have your being the sea. The sea is within you and without you, and you are made of sea, and you will end in sea. The sea surrounds you as your own being."
So the only true answer to the question "What is Zen?" is the one that you find for yourself.