"Junk science" is simply a negative term for any kind of science that we don't like. There are as many definitions of junk science as there are people who use the term.
There are many questions that we can ask about scientific research, particularly when applied to questions of public policy. Research claims may not be based on scientific principles, or they may be based on careless or even fraudulent research. Or the studies themselves may be sound, but they may be misinterpreted by those who cite them. Or, even when studies are sound and reasonably well understood, there may be disagreements about the implications for policy decisions.
Labeling a piece of scientific research, or political advocacy, "junk science" generally adds more heat than light to the debate.
Although the term "junk science" is used by many people, for many reasons, the Junk Science movement is a focused and well-funded campaign to use the "junk science" label to discredit specific points of view, particularly a wide range of environmental concerns. Proponents of the anti-environmental Junk Science movement are sometimes referred to as "junketeers".
The founder of the Junk Science movement is Peter Huber, who popularized the term in the late 80s and early 90s in support of efforts to "reform" the tort system. Huber wrote several books and articles, most notably Galileo's Revenge arguing, almost exclusively from anecdotal evidence, that the tort system was out of control and that courts were being increasingly victimized by misleading "junk science". Kenneth J. Chesebro wrote an excellent analysis of Galileo and Huber's other tort-reform writings in Galileo's Retort.
Perhaps the best known junketeer is Steven Milloy, who runs the web site junkscience.com and refers to himself, quite appropriately, as the Junkman. Milloy is a talented and prolific "spinner" whose motto is apparently "what we don't know can't hurt us": the repeated theme in his work is that many environmental risks are uncertain, the implication (delivered unsubtly, with ample helpings of insulting language) is that they therefore don't matter.
Junk Science links
Project Whitewash home page