Authoritative Definitions (Answers.com):
"Authoritative" is an interesting word whose roots have both positive and negative connotations. To be considered the authoritative source on a subject is certainly a positive thing. Authoritarian on the other hand has negative connotations.
The nature of "authoritative" is shifting with the changes brought on by the web. In the past, the authoritative source, in the form of a person, was the person one went to to find the answer on a particular topic. Usually this person had a great deal of experience and knowledge, and/or special access to research materials. As the web makes more and more material directly available the need to rely on intermediaries decreases. One doesn't need to consult the authority in the field if one can quickly and easily find the same information using the web. This is a difficult adjustment for those who place value on being viewed as authoritative.