Answer Description

The term Trusted Operating System (TOS) refers to an operating system that has been certified to have a certain level of security. The requirement of this certification are defined in the Common Criteria for Information Technology Security Evaluation (abbreviated as Common Criteria or CC). Many organizations (especially governments) may only use operating systems certified as Trusted OS's.

Wikipedia

Trusted Operating System (TOS) generally refers to an operating system that provides sufficient support for multilevel security and evidence of correctness to meet a particular set of government requirements. The most common set of criteria for trusted operating system design is the Common Criteria combined with the Security Functional Requirements (SFRs) for Labeled Security Protection Profile (LSPP) and mandatory access control (MAC). The Common Criteria is the result of a multi-year effort by the governments of the U.S., Canada, United Kingdom, France, Germany, the Netherlands and other countries to develop a harmonized security criteria for IT products.