Similarly to the question Who first coined the term Artificial Intelligence?, who first coined the term "artificial general intelligence"?
According to Ben Goertzel, the first person that probably used the term "artificial general intelligence" (in an article related to artificial intelligence) was Mark Avrum Gubrud in the 1997 article Nanotechnology and International Security. Here's an excerpt from the article.
By advanced artificial general intelligence, I mean AI systems that rival or surpass the human brain in complexity and speed, that can acquire, manipulate and reason with general knowledge, and that are usable in essentially any phase of industrial or military operations where a human intelligence would otherwise be needed. Such systems may be modeled on the human brain, but they do not necessarily have to be, and they do not have to be "conscious" or possess any other competence that is not strictly relevant to their application. What matters is that such systems can be used to replace human brains in tasks ranging from organizing and running a mine or a factory to piloting an airplane, analyzing intelligence data or planning a battle.
Note that the term "AGI" could have been used even before that Mark A. Gubrud's article (as Goertzel also suggested).
In any case, Ben Goertzel help to popularise this term, especially with the book Artificial General Intelligence. AGI was previously known as "strong AI", which goes back to John Searle's Chinese Room argument, although strong AI often refers to an AGI with consciousness, and the definition of AGI doesn't necessarily imply consciousness.
You can read more about the history of the term "AGI" in this Goertzel's blog post.
According to this Wikipedia article
The term "artificial general intelligence" was used as early as 1997, by Mark Gubrud. in a discussion of the implications of fully automated military production and operations. The term was re-introduced and popularized by Shane Legg and Ben Goertzel around 2002.
The research objective is much older, for example Doug Lenat's Cyc project (that began in 1984), and Allen Newell's Soar project are regarded as within the scope of AGI.