In Active Directory, what defines how security principals from one domain can access network resources in another domain?

Trust Relationship defines how security principals from one domain can access network resources in another domain.

Explanation:

When a logical link is established between two domains, it is known as trust relationship. These domains are divided into two parts, the trusting domain, and the trusted domain. For the trust relationship to take place, the trusting domain has to accept the login authentication of the trusted domain.

Also, the users are considered within the trusted domain, while the resources are withing the trusting domain.

Trust can be categorized as manual or automatic, or transitive or non-transitive.