There was a recent discussion about the term "trust" in the context of trust federation. Jane Winn gave me a hint on a good paper by Timothy W. Guinnane that points out that the discussion about the meaning of trust is superfluous: "the useful parts of the idea of trust are implicit in older notions of information and the ability to impose sanctions".

Just for the record a few definitions that come up in the thread:

"Willingness to engage in a transaction"
"Reliance on the enforcement of a certain policy" 
"Understood probability in the expected behavior of another party".

There are more scientific definitions of trust available, like in the Wiki article on computational trust: 
http://en.wikipedia.org/wiki/Computational_trust#Defining_Trust