Declarative security is a syntax that uses attributes to place security information into the metadata of the code.
Declarative security is a syntax that uses attributes to place security information into the metadata of the code. The attributes indicate the type of privileges required by an assembly, a class, or a certain member of the class. The common language runtime retrieves these attributes and informs the runtime security system about the required permissions.
Links:
MSDN article on Declarative Security