I think that's another one of those things that everyone finds difficult to agree on.  The safest definition for me would be deliberate and repeated transactions with no intention to transfer any value, but it's not always easy to recognise such transactions if the culprit is determined to cover their tracks.  Some attackers are more blatant than others.  But equally it's easy to lose context and assume that all small value transactions or transactions with low fees are spam, but this isn't a safe assumption due to users in less economically wealthy parts of the world getting involved.  All we can really do is minimise the motivation to engage in deliberate spamming by making it expensive or difficult (or both) to do.

Was Litecoin's spam fix ever implemented in Bitcoin?  And if not, could we look at implementing that as part of this proposal?