Zen is a school of Mahayana Buddhism that developed in China during the 6th century as Chán. From China, Zen spread south to Vietnam, northeast to Korea and East to Japan.
The word Zen is derived from the Japanese pronunciation of the Middle Chinese word 禪 (dʑjen) (pinyin: Chán), which in turn is derived from the Sanskrit word dhyāna, which can be approximately translated as "absorption" or "meditative state". (Source: Wikipedia)
One of the great zen stories:
A Cup of Tea
Nan-in, a Japanese master during the Meiji era (1868-1912) received a university professor who came to inquire about
Nan-in saved tea. He poured his visitor's cup full, and then kept on pouring.
The professor watched the overflow until he no longer could restrain himself. 'It is overfull. No more will go in!'
‘Like this cup,' Nan-in said. ‘You are full of your own opinions and speculations. How can I show you Zen unless you
first empty your cup? '