Encyclopedic Dictionary of Roman Law, Adolf Berger.

This book was originally printed in 1953 and became unavailable. It was reprinted in 1980 and 1991 but is once again out of print. I found a copy of it on ebay, but there are used copies available through Amazon also.

This is a wonderful reference tool. It's set up exactly like a dictionary, which you might expect from the title. It's very easy to use and is an excellent starting point for research.