Trust (of a party in X) is the (un)conscious decision by that party that X is in fact the case. X can be anything. To trust is to believe to the extent that one can rely on X to be the case. One can trust a president - e.g. to make the right decisions. One may trust (rely on) one's own judgements or conscience.
Trust is distinct from trust level, which is a (subjective) degree of belief or confidence that a party has in (the truth of) X. In order to trust X, the associated trust level of X would need to have exceeded some threshold, which typically depends on the consequences the party would like it to have. For example, a party may associate some trust level to the statement "Mr Salesmann will not sell me junk" The threshold for that party actually trusting that this is in fact the case is typically lower when the party buys a pear from Mr Salesmann, than when it would buy a house from Mr Salesmann.
Trust is not something that is given, but something that parties (un)consciously, and continually decide about - it changes over time. Since parties are autonomous, their trust is highly subjective. As a consequence, the idea of having 'trusted registries', 'trusted issuers' that do not take this subjectivity into account basically act as (centralized) authorities, is a denial of the fact that parties are autonomous. While we acknowledge that such ideas (have a right to) exist, we do not follow them.
In many practical situations, trust has a few basic issues, in that it is often not clear