trade secret

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A secret formula, method, or device that gives one an advantage over competitors.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun business A formula, practice, process, design, instrument, pattern, or compilation of information used by a business to obtain an advantage over competitors within the same industry or profession.
• noun business, law A formula, practice, device, idea, process, design, instrument, pattern, or compilation of information which is not patented and which is used by the owner to obtain an advantage over competitor within the marketplace and is treated in a way that can reasonably be expected to prevent the public or competitors from learning about it except when there is improper access, eavesdropping, data acquisition or theft (where the country's law recognises theft) of that trade secret and where the implementation of the trade secret does not allow reverse engineering.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a secret (method or device or formula) that gives a manufacturer an advantage over the competition