a trade secret
1. Literally, a particular way of making something that a company keeps secret from competitors. The recipe for our famous rib sauce has been a trade secret for decades.
2. By extension, any secret one keeps about the way one makes or does something. A: "How do you get the colors in your pictures to turn out so brilliantly?" B: "Sorry, trade secret."
a ˌtrade ˈsecret
1 a secret about a particular company’s method of production: The ingredients of Coca-Cola are a trade secret.
2 (humorous) a secret about how you make or do something: ‘Can I have a recipe for this cake?’ ‘No, you can’t. It’s a trade secret.’