This could be a testing question for advanced students – the master might say, “Say a word of Zen!” to test the student's realisation.
But that's not what we're doing here (whew!). Literally, Zen means concentration or meditation. The Sanskrit word Dhyana, meaning one-pointedness of mind, became Chan in Chinese, which was then transliterated as Zen when this school came to Japan. Chan or Zen, although originating in India, spread to China, Vietnam, Korea and Japan – and then more recently to the West.
Zen is said to have started when Shakyamuni Buddha (the historical Buddha) held up a flower in front of an assembly, and only Mahakashyapa 'got it' and smiled. He thus became the first Zen ancestor after Shakyamuni. From Shakyamuni down to present-day Zen teachers, the Dharma has been handed down through face-to-face transmission teacher to student through eighty-odd generations.
Indeed, Zen is talked of as a 'special transmission outside words and scriptures' despite there being stacks and stacks of writing about it; this indicates that you can read all you like, but unless you practise zazen (Zen meditation), none of that is going to enlighten you.
When Bodhidharma first brought the Zen teachings from India to China, he had an interview with Emperor Wu, who was a devout Buddhist, and had built several monasteries and supported the development of Buddhism in China. He asked Bodhidharma what merit would accrue to him from these actions, to which Bodhidharma replied, “None at all.” Taken aback by this, the emperor asked, “So what is this teaching that you are bringing, then?” Bodhidharma replied, “Vast emptiness, nothing holy.”
Even more puzzled, Wu asked, “Who are you?”
Bodhidharma replied, “I don't know” – or, as another translation has it, “Not-knowing is here.”
We can summarise by saying that Zen is about 'vast emptiness, nothing holy' which is only accessible when in a state of 'don't-know mind'.
In terms of what's happening in the West today, mindfulness is much better known than Zen, although the word 'zen' tends to be used as a shorthand for very relaxed, and, in the world of interior design, rather spacious and uncluttered. Which hopefully reflects our minds when we've been practising for a while.
I think of mindfulness as a sort of 'gateway drug' to Zen, because it is certainly a part of Zen, maybe the first step – to learn, through meditation, that the endless stream of stuff that goes through our minds is neither the truth, nor who we really are. Learning not to be so caught up in our own thoughts certainly improves our lives, if only by allowing us to relocate back into our senses, which of course is where all the nice stuff happens, and we get to appreciate life much more.
However, Zen is part of the Mahayana ('Great Vehicle') Buddhism – though you don't have to be a Buddhist to practise it – and in the Mahayana it is not just about what happens in this 'skin bag' because there are others on the planet who also need taking care of. So Zen is also about going beyond self, partly by realising that ultimately we are one with it all anyway, so helping another is like the left hand helping the right. As the old joke has it:
“What did the Zen student say to the hot dog man? Make me one with everything!”