What is the legal definition of a "red light violation" in Cambridge,
MA? What constitutes running a red light, and where is this
information found (what document in state or city law)?

Greetings laban!

There are lots of great links on the Massachusetts government and
related sites about traffic law and information relating specifically
to traffic signals that should be able to answer your question. 
Specifically these three links in relation to each other (I list all
links at the end of the answer also):

http://mass.gov/rmv/dmanual/102004/chapter4.pdf

http://www.mass.gov/courts/courtsandjudges/courts/districtcourt/cmviassess.pdf

http://www.mass.gov/legis/laws/mgl/89-9.htm


Some good alternate search terms would be "red signal" and "traffic
signal".  Below, I have listed a bunch of links that are related to
the information you are looking for.  Much of it is related directly. 
There are also links to Massachusetts general law.  I have provided
both table of contents and search pages.

Please let me know if I can clarify any of this information for you.

Thanks for the interesting question!

Thinkout

Search Strategies in Google:
traffic violation cambridge, ma
"red signal" massachusetts
"traffic signal" massachusetts
"traffic law" massachusetts

Here are some useful links that I found.

Drivers manual.  Contains the rules of the road.
http://www.mass.gov/rmv/dmanual/index.htm


Drivers manual, rules of the road chapter:
http://mass.gov/rmv/dmanual/102004/chapter4.pdf


Document that shows traffic light violations 
and additional information look right at the top:
http://www.mass.gov/courts/courtsandjudges/courts/districtcourt/cmviassess.pdf


General Laws of MA.  This section talks about red lights a little:
http://www.mass.gov/legis/laws/mgl/89-9.htm


General Laws of MA.  This section talks about turning on red lights;
http://www.mass.gov/legis/laws/mgl/89-8.htm


General Laws of MA.  Chapter Listing so you can look up what you need.
You can use the links at the bottom of the page to move forward and
backward through the table of contents:
http://www.mass.gov/legis/laws/mgl/gl-89-toc.htm


This link allows you to search through the laws using keywords:
http://www.mass.gov/portal/index.jsp?query=chapter+89+section+9&startAtResultNum=1&pageID=mg2searchlanding&sid=massgov2&numResults=50&collectorName=LEGxLAWSxMGLx