There is a general definition that you will find in dictionaries and that is generally accepted. So I think there is a definition that everyone agrees on when there are not pushing an agenda.

Then there is the use of the term for rhetorical and political purposes... This is what obviously varies according to the interests of those using the term. Obviously the terrorists are always the 'other guys'.

Note that the definition I gave, which is nothing more than the Oxford dictionary one, includes a crucial word, "unlawful". This is very important to differentiate between warfare and terrorism. A sovereign state uses violence for political aims, this is warfare and it is 'lawful' (it may be unlawful under international law, though). This is why action by nation-states are usually not labelled 'terrorism'.