A couple of answers mention the LD_RUNPATH environment variable:
- use RPATH but not RUNPATH?
 - How to blacklist a shared library to avoid it being fetched by the loader?
 
but I couldn't find any description or official reference, including in the ld(1) and ld.so(8) Linux man pages. It seems that the goal is to have a lower precedence than LD_LIBRARY_PATH, particularly useful under Solaris, where LD_LIBRARY_PATH overrides everything.
A search on Google shows some discussions involving LD_RUNPATH, but nothing concrete. A search on Wikipedia is also unsuccessful. I also did some compilation/run tests with shared libraries under GNU/Linux and Solaris, but couldn't notice any effect.
So, what is this environment variable? Is there any official documentation?