The most obvious (and commonly given) answer to this question is John McCarthy, but I have traced the first usage of this term to a 1955 paper, which McCarthy co-authored with Marvin Minsky, Nathaniel Rochester, and Claude Shannon. Although McCarthy was the lead author and probably the main organizer of the 1956 Dartmouth Summer Research Project on Artificial Intelligence, it could just as easily have been that one of these co-authors actually proposed the term "artificial intelligence". Does anyone know of any other evidence to suggest whose idea the actual term was? All of these individuals are now dead, but maybe some of their close associates or former students might be able to also shed light on the question.
Based on my research, there is no conclusive answer as to who coined the term, although McCarthy can be credited with helping the term to find widespread acceptance within the computer science community. See "AI: The Tumultuous History of the Search for Artificial Intelligence" by Daniel Crevier (Published in 1993 by BasicBooks):
"Overcoming the resistance of some participants (Samuel felt that "artificial" sounded phony, and Newell and Simon persisted in calling their work "complex information processing" for years afterward), McCarthy persuaded the majority to go for "artificial intelligence." He lays no claim to having coined the phrase, and admits it may have been used casually beforehand. Yet nobody denies him the achievement of getting it widely accepted."
-- Daniel Crevier, p.50