Historically, various different notions of trust can be found, each addressing particular aspects of ICT systems, e.g., trust in electronic commerce systems based on reputation and recommendation, or trust in public key infrastructures. While these notions support the understanding of trust establishment and degrees of trustworthiness in their respective application domains, they are insufficient when addressing the more general notion of trust needed when reasoning about security in ICT systems. Furthermore, their purpose is not to elaborate on the security mechanisms used to substantiate trust assumptions and thus they do not support reasoning about security in ICT systems. In this paper, a formal notion of trust is presented that expresses trust requirements from the view of different entities involved in the system and that enables to relate, in a step-by-step process, high level security requirements to those trust assumptions that cannot be further substantiated by security mechanisms, thus supporting formal reasoning about system security properties. Integrated in the Security Modeling Framework SeMF this formal definition of trust can support security engineering processes and formal validation and verification by enabling reasoning about security properties with respect to trust.