You can shop around for definition attempts.
... on the websites of SLE conference series.
... in the guest editors' introduction to the IEEE TOSEM 2009 special issue on SLE.
As a matter of fact, we don't have a proper, generally accepted and adopted definition.
This indeed also showed at last year's SLEBOK workshop at SLE 2012.
There is quite a few people (including me) who think that this needs to be fixed.
On a related account, it is not even particularly clear what the core or extended set of SLE concepts would be. This makes it hard to properly cover the subject in teaching. I somehow have managed in my SLE class, but I do not claim to be very consistent and palatable as to the use of terminology (ontology) in the course.
This semester's edition of my SLE course will feature reading (and corresponding presentations and discussions by the students) with one reading slot focused on basic SLE literature. I have started to collect a richly annotated bibliography of "fundamental SLE papers", which I released at OOPSLE 2013 (at WCRE 2013) in Koblenz earlier this week.
Please have a look:
I hope it may be useful for others.
I also look forward collaboration on the subject.