What is Natural Language Understanding?

I've read lots of papers and seen lots of talks that justify a task as being useful to doing "natural language understanding." The problem I have here is that I really have no idea what NLU actually is. I think the standard line is that NLU is the process of transforming natural language text into "some representation" that is "easy" for a computer to "manipulate." Of course, the quoted words are so vacuous in meaning that I can easily imagine reasonable definitions for them that make this task either trivial or essentially impossible.

What I find unfortunate here is that, without trying hard to pin down definitions, this seems like a completely reasonable goal for NLP technology; in fact, my impression is that up until 10 or 20 years ago, this actually was one of the major goals of the field. According to the anthology, roughly 25% of the papers in 1979 contained the terms "NLU" or "language understanding", compared to 20% in the 1980s, 10% in the 90s and 7% in the 2000s. One possible explanation of the dwindling of this topic is that publication has become increasingly driven by experimental results, and if one cannot pin down a definition of a task, one cannot reliable compare results across multiple papers.

The recent push on textual entailment bears some semblance to NLU, but is simultaneously better defined and more restricted (though I admit to having heard some grumbling that some more work needs to be done to get textual entailment to be a very well defined task). There also continues to be a modicum of work on database filling and natural langauge database queries, though certainly not at the same rate as before.