About
A secret is a cryptographic material that should not be known by the public in order to secure the transaction.
A secret is categorized as something you have authentication.
It's also known as the private (to convey that you should not share it at all).
Scheme
Asymetric
In a asymmetric scheme, this is the private key while the public key can be shared.
When you read secret in a asymmetric cryptographic documentation, it has a lot of chance that it refers to it.
Symmetric
In a symmetric scheme, the secret is the symmetric encryption key (or shared secret). It should only be shared between the two parties, not publicly.