Trust is an imprecise, relative and subjective term in an objective and scientific discipline. It doesn't really fit in information security -- but it's the best we've got. It is used ubiquitously to imply 'secure'. Since 'absolutely secure' is something we strive for but cannot achieve, we use instead the imprecise word 'trusted'. It allows us to claim that the product is safe without having to guarantee it is secure. A trusted system is to the best of our knowledge and belief a secure system (but we're not guaranteeing it).
An illustration of this can be found in the usual definition of evaluation (even our own): a method for measuring trust. Strictly speaking, how can you formally measure the degree of an imprecise concept? But despite this, we know what is meant.