I think you need the music dictionary to look up chamber orchestra. I found some excerpts near the bottom of this page:

http://www.shilakowskyarts.com/professorharrisshil.htm

Basically it says that it used to mean a group where each instrument had a part (rather than being divided up into sections). The definition (apparently taken from the Oxford Dictionary of Music) goes on to say:

There are organizations calling themselves 'Chamber Orchestra' that are merely small ordinary orchestra. The term is a new one and has not yet settled down to precise meaning...."

So there you have it. FWIW.