a piece of information about a product that is known only to the particular company that makes it

The exact ingredients of Coca-Cola are a trade secret. 可口可乐的确切成分属于商业秘密。

a piece of information that you are not willing to tell anyone

My aunt won't tell anyone her age - she says that it's a trade secret. 我姑妈不愿意把年龄告诉任何人，她说这是个秘密。