In the security engineering subspecialty of computer science, a trusted system is one that is relied upon to a specified extent to enforce a specified security policy. This is equivalent to saying that a trusted system is one whose failure would break a security policy (if a policy exists that the system is trusted to enforce).
The word "trust" is critical, as it does not carry the meaning that might be expected in everyday usage. A trusted system is one that the user feels safe to use, and trusts to perform tasks without secretly executing harmful or unauthorized programs; trusted computing refers to whether programs can trust the platform to be unmodified from the expected, and whether or not those programs are innocent or malicious or whether they execute tasks that are undesired by the user.
A trusted system can also be seen as a level-based security system where protection is provided and handled according to different levels. This is commonly found in the military, where information is categorized as unclassified (U), confidential (C), secret (S), top secret (TS), and beyond. These also enforce the policies of no read-up and no write-down.