What you're looking for is not a word, but rather a phrase, it's "Je ne sais quois." The literal translation from French to English of Je ne sais quois is "I know not what."
It refers to something that can't be described adequately using just words, it might be used when something transmits a feeling or emotion. One might describe a piece of artwork as not being perfect "yet it still has a certain 'Je ne sais quois' about it," something that makes it special.
In Chicago the HeraldOnline is reporting about a food event taking place that's "part mystery, part flash feast, part Je ne sais quois," calling it "the most sought after secret event."