Getting Things Done: "Official" Definition
David Allen has a go at pinning it down. Now, in one sense, since he wrote the classic book, he has every right to try to control the definition.
On the other hand, GTD is beloved of the disorganized and creative. Often these people are fairly relaxed about allowing terminology to wander and extending ideas through metaphors and analogies but still aspire to a little more self-control.
Possibly a fairly loose definition of GTD has been part of its success. People think they know what it's about, but are really mapping it onto ideas they received second and third hand, and which they've personally remixed with other intuitions. Maybe they'll prefer a fairly loose notion.
Having said all that, the actual definition Allen gives waffles like Belgium. So maybe he knows his market pretty well ;-)