According to my dictionary, greed is defined as: "intense and selfish desire for something, esp. wealth, power, or food". Of course, by itself, this definition has no negative meaning; it's just as possible to be rationally greedy for something as irrationally greedy for it. However, it's become common to attach a negative connotation to it, just as is common with "selfish".
While I don't think there's an official Objectivist definition for "greed", one can infer from how Ayn Rand used it that she did not accept the negative connotations attached to the word. Indeed, chapter 2 of section 2 in Atlas Shrugged was entitled: "The Utopia of Greed". There are many examples of the looters using the word in the pejorative sense (e.g., James Taggart says: "Ellis Wyatt is a greedy bastard who's after nothing by money"), but there are also a number of examples where she makes it clear that her protagonists reject that definition (e.g., Dagny's first dinner in the valley has several references).