I have wondered from time to time about the phrase "attorney at law." Are there other kinds of attorneys? Attorneys at arms? If not, why do we specify?
One definition of an attorney is "A person appointed to act for another in business or legal matters." See also http://www.merriam-webster.com/dictionary/attorney. This usage is seen in such phrases as "power of attorney", which employs an attorney-in-fact.
Attorney in the US sense is an abbreviation for attorney at law, or public attorney. There are different types, like the private attorney.
Source: Online Etymology Dictionary