Trust is a measure of how much an entity relies on other entities for its security.
A principal in a system with security trusts another principal if it allows its security to depend on assertions made by that other principal. For example, when a web browser assumes that a web site is legitimate because the site shows a certificate signed by a certificate authority, the browser trusts the CA. Trust is often established and propagated through cryptography. The concept is derived from the plain English meaning of the word, which is studied by social sciences.
If your question is about determining what security policy should apply or whether a system meets a desired security policy, and it doesn't involve code, consider asking on Security Stack Exchange.
