For something like a century, scientists, philosophers of science, and many other scholars have grappled with this question: What criteria, principles, rules, or behavior characterize science by contrast to all other things? What exactly is “not-science”, in other words? What exactly is “pseudo-science”?
The upshot of these many decades of suggestions and discussions and argumentation among the most well-informed specialists is that
NO COMBINATION OF CRITERIA, PRINCIPLES, RULES, or BEHAVIOR
CAN DISTINGUISH SCIENCE UNEQUIVOCALLY
from NON-SCIENCE or from “PSEUDO-SCIENCE”
The classic summation of failed attempts is Larry Laudan’s “The demise of the demarcation problem” . Even those who don’t agree that the issue is at a dead end  attempt to find a practical distinction by means of “family resemblances” or “fuzzy logic”, thereby acknowledging that the distinction can only be approximate, probabilistic, never a definitive one: no hard-and-fast, unequivocally valid set of criteria is able to identify an instance of “pseudo-science” without delving into the particularities of methods, evidence, and inference specific to that instance. Of course you can declare something wrong if you can show the methods to be inappropriate or incompetent, or that the claimed evidence is fudged or faulty or incomplete, or inferences are drawn against logic. But you don’t need a general, universal definition of “pseudo-science” to do that.
By hindsight, it even seems obvious that no universal definition of “science” could be found. It would have to be based on what everyone agrees constitutes science: biology, chemistry, geology, physics, etc. — not to speak of the behavioral and social sciences. Inferring from those real-world enterprises the “essence” of science means educing or inducing universal characteristics from empirical observations. But philosophy has long understood that induction from empirical observation or experience can never be guaranteed to yield universally applicable generalizations. (The classic illustration is that empirical observation yielded the principle that all swans are white, which was confounded upon the discovery of black swans in Western Australia.)
Moreover, a universally applicable definition of science would not change over time, whereas the activities that everyone calls “science” have changed drastically over time . Most pertinent: some matters once accepted as proper science later became generally regarded as not-science or even pseudo-science, and some matters once pooh-poohed as pseudo-science later became accepted as quite proper mainstream science, for example, electromagnetic phenomena in biology .
The term “pseudo-science” can only mean something that pretends to be science but isn’t; and since there is no valid definition of “science”, there is equally no valid definition of “pseudo-science” by which it could be recognized.
Nevertheless, it remains quite common in public discourse that practicing scientists as well as professional and amateur pundits use the epithet “pseudo-science” to malign specific claims (say, the existence of Loch Ness Monsters or of Bigfeet) or even whole fields of activity (parapsychology, “cold fusion”, cryptozoology, ufology, etc. etc.)
The basis for such maligning and pooh-poohing is that the topic has been found wanting by the prevailing consensus in mainstream science. But that basis is fatally flawed: the history of science tells of one after another mainstream consensus being itself found wanting and replaced, often by something that the mainstream had earlier resisted vigorously or ignored studiously [5-8].
The state of the intellectual art about this has been quite plain for decades. But this intellectual art is the domain of history of science, philosophy of science, sociology of science, and the comparatively young interdisciplinary umbrella of STS (Science & Technology Studies), of which most scientists, journalists, and pundits generally are woefully ignorant; an ignorance that extends perforce to the public media generally, and to Internet punditry, very much including Wikipedia and its ilk, to an extent that would be highly embarrassing if those people and groups knew even a smidgeon of what they ought to before blathering about “pseudo-science” or “science”.
There is so much of this ignorant blathering that I usually ignore it, but that blissful state was interrupted when I became aware of a recent instance from the prominent and prestigious TED and its franchised TEDx ventures, which bill themselves as promoters of high-quality seminars — “Ideas worth spreading . . . the power of ideas to change attitudes, lives and, ultimately, the world”.
What TED and TEDx spread about science and pseudo-science is ignorant rubbish (A letter to the TEDx community on TEDx and bad science). As with other charlatans, they know how to cover their tracks: They acknowledge reality correctly in sweeping general statements and then try unobtrusively to get around it:
“What is bad science/pseudoscience? There is no bright and shining line between pseudoscience and real science”.
RIGHT. But that valid statement is followed immediately with tiptoeing away from validity:
“Needless to say, this makes it all terribly hard to detect and define”.
NO: it makes it IMPOSSIBLE to detect as a genre or class or supposed exemplar of a genre or class. The only way to evaluate any counter-mainstream claim is to dig into the specific particularities, and then to concede that any contemporaneous judgment of plausibility or potential validity can only be probabilistic. That’s the clear lesson of centuries of history of science and a century or so of scholarly preoccupation with this issue .
The TED ignoramuses then proceed to offer “guidelines” for what constitutes “good science”. All of those “guidelines” are plainly misguided, reflecting a childishly naïve, uninformed view of science:
“It makes claims that can be tested and verified”
Every scholarly source since Popper’s proposal of “falsifiability” has been clear about the impossibility of verification — there can never be a guarantee against the future appearance of a “black swan”.
“It has been published in a peer reviewed journal (but beware… there are some dodgy journals out there that seem credible, but aren’t.)”
As Ziman pointed out , something like 90% of the primary research literature is wrong to some degree (in physics, but that’s the epitome of science and it may well be worse in other fields)
“It is based on theories that are discussed and argued for by many experts in the field”
History teaches that all the experts can be wrong — and are wrong in the longest run.
“It is backed up by experiments that have generated enough data to convince other experts of its legitimacy”
That the experts agree is no reason to believe them, in part because in the long run they’re usually wrong [5-8]. Here’s a nice way of putting it :
“Historically, the claim of consensus has been the first refuge of scoundrels; it is a way to avoid debate by claiming that the matter is already settled. Whenever you hear the consensus of scientists agrees on something or other, reach for your wallet, because you’re being had. . . .
Consensus is invoked only in situations where the science is not solid enough.
Nobody says the consensus of scientists agrees that E=mc2. Nobody says the consensus is that the sun is 93 million miles away. It would never occur to anyone to speak that way”.
“Its proponents are secure enough to accept areas of doubt and need for further investigation”
Few mainstream scientists exhibit that quality, as anyone familiar with actual scientists or with the history of science or the sociology of science knows
“It does not fly in the face of the broad existing body of scientific knowledge”
The most significant advances are those that do contradict contemporary views; they spark scientific revolutions and become praised only by hindsight [5-8]
“The proposed speaker works for a university and/or has a PhD or other bona fide high level scientific qualification”
Any number of incompetents and kooks have such qualifications, as even a brief participation in a research community makes evident.
It is an endless source of astonishment to me that totally uninformed, ignorant people feel so free to hold forth with arrogant assurance, as TED does on the issue of science and pseudo-science. Don’t the TEDdies and their ilk ever stop to wonder where their knowledge comes from? “Knowledge” that is actually abysmal ignorance?
 Pp.111-27 in Physics, Philosophy and Psychoanalysis, ed. R. S. Cohen & L. Laudan, Dordrecht: D. Reidel, 1983
 For example, Philosophy of Pseudoscience: Reconsidering the Demarcation Problem, ed. M. Pigliucci & M. Boudry, University of Chicago Press, 2013
 Henry H. Bauer, Three Stages of Modern Science, Journal of Scientific Exploration, 27  505-13; From dawn to decadence: The three ages of modern science
 Henry H. Bauer, Science or Pseudoscience: Magnetic Healing, Psychic Phenomena, and Other Heterodoxies, University of Illinois Press, 2001
 Bernard Barber, Resistance by scientists to scientific discovery, Science, 134 (1961) 596-602
 Ernest B. Hook (ed)., Prematurity in Scientific Discovery: On Resistance and Neglect, University of California Press, 2002
 Thomas S. Kuhn, The Structure of Scientific Revolutions, University of Chicago Press, 1970
 Gunther Stent, “Prematurity and uniqueness in scientific discovery”, Scientific American, December 1972, 84-93
 John Ziman, p. 40 in Reliable Knowledge: An Exploration of the Grounds for Belief in Science, Cambridge University Press, 1978
 Michael Crichton, “Aliens Cause Global Warming”, Caltech Michelin Lecture, 17 January 2003