Rob Blackie Influence is most usefully thought of as the ability to change what somebody thinks or does.
Marshall Manson: Influence is the propensity of an individual to alter the views or behaviour of another individual or a community. It is absolutely critical to note that popularity and influence are not the same.
James Warren In simplistic terms, someone who is influential is trustworthy (as relates to the topic in hand), is willing to share an opinion and has access to a popular channel to do so. Of course, the reality is far more complex.
Michael Darragh Influence goes hand in hand with relevance. It is not enough just to have the reach to tell your story. The message demands meaning and resonance if it is to have any real effectiveness.
Dirk Singer It is a mix of reach and engagement - to what extent is anyone motivated to act by what they read or hear about. In the case of blogs, this would be the number of other websites that take content from and link into you.