I think the term simply means what it says... someone who advises you about your taxes.
That's not a very helpful answer or response, though, so let me try to throw out a couple three additional comments.
First, you might be interested in who you can employ or hire to represent you before the IRS... and the simple answer to this question is an attorney can, a CPA can and an enrolled agent can. For more detailed information about this (including exceptions to this generalization), you can refer to circular 230 available here: http://www.irs.gov/pub/irs-utl/pcir230.pdf
Second, a CPA or attorney doesn't necessarily possess specialist or expert knowledge in taxes. Lots of attorneys and CPAs (maybe most?) specialize in an area other than tax accounting or tax law. Enrolled agents, in contrast, should usually be tax specialists since that's their own deal. I would say therefore that you want a tax attorney or tax CPA or EA giving you advice.
Third, though circular 230 which is a pretty authoritative document says that only attorneys can practice law (see paragraph 10.31) , I think as a practical matter everybody agrees that you can get tax law advice not only from an attorney but also from a CPA and EA. See paragraph 10.32 for example. BTW I tried to find the joint statement of the American Bar Association and American Institute of CPAs that I think exists... but could not locate that. (Heck... some CPAs and EAs can represent you before the tax court.)
Hope that helps.