This word permeated pop culture when the philosophical book "Zen and the Art of Motorcycle Maintenance" was published in 1974. Zen is actually a branch of Mahayana Buddhism, which developed in China in the sixth century and later spread to Japan. It teaches that meditation and simple, personal reflection is the path to spiritual enlightenment and that preoccupation with material things and selfish desires often create discomfort and mental anguish. Current usage has transformed the word into an adjective describing anything with these qualities.
Example: Justin’s client had a very high-stress job as a neurosurgeon, so he created Zen interiors for her home, providing her a sanctuary that allows her to calm her mind after being on call and performing life and death procedures for hours at a time.