I quote from the Wikipedia article.
The word paradigm comes from the Greek word παράδειγμα (paradeigma) which means "pattern" or "example", thus the purists, including my Concise Oxford Dictionary, give this as its meaning, but usage has expanded the meaning. The 1900 Merriam-Webster dictionary defines its technical use only in the context of grammar or, in rhetoric, as a term for an illustrative parable or fable.
Philosopher of science Thomas Kuhn gave this word its contemporary meaning when he adopted it to refer to the set of practices that define a scientific discipline during a particular period of time. Kuhn defines a scientific paradigm as:
what is to be observed and scrutinized; the kind of questions that are supposed to be asked and probed for answers in relation to this subject; how these questions are to be structured; and how the results of scientific investigations should be interpreted.
Thus, within normal science, the paradigm is the set of exemplary experiments that are likely to be copied or emulated. The prevailing paradigm often represents a more specific way of viewing reality, or limitations on acceptable programs for future research, than the much more general scientific method. A more disparaging term groupthink, and the term mindset, have very similar meanings that apply to smaller and larger scale examples of such disciplined thought. A simplified analogy for paradigm is box in the commonly used phrase "thinking outside the box". Thinking inside the box is analogical with normal science. The box encompasses the thinking of normal science and thus the box is analogical with paradigm.
Here is how the current physics pardigm is explained in Wikipedia: An example of a currently accepted paradigm would be the standard model of physics. The scientific method would allow for orthodox scientific investigations of many phenomena which might contradict or disprove the standard model; however grant funding would be more difficult to obtain for such experiments, in proportion to the amount of departure from accepted standard model theory which the experiment would test for. For example, an experiment to test for the mass of the neutrino or decay of the proton (small departures from the model) would be more likely to receive money than experiments to look for the violation of the conservation of momentum, or ways to engineer reverse time travel.
So the reports of 'cold fusion' a few years ago were so offensive to the physics community because they challenged the prevailing paradigm.
But the previous physics paradigm was different. Here's Wikipedia again:
In 1900, Lord Kelvin famously stated, "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement." Five years later, Albert Einstein published his paper on special relativity, which challenged the very simple set of rules laid down by Newtonian mechanics, which had been used to describe force and motion for over three hundred years. In this case, the new paradigm reduces the old to a special case (Newtonian mechanics is an excellent approximation for speeds that are slow compared to the speed of light).
The change from the pre-1900 paradigm to Einsteinian physics is known as a paradigm shift. Kuhn's idea was that scientific knowledge proceeds in just this way; there is no gradualist pecking away at a theory until it metamorphoses into a new one. An old paradigm is replaced by a new one in a relatively short space of time.
Interestingly, just such a mechanism was proposed for the theory of evolution by Gould and Eldredge. It has been accepted by evolutionary theorists that one species changed to another by the gradualist accumulation of small mutations. Gould and Eldredge believed that change happened in a series of large jerks, which produced what Goldschmidt described as 'hopeful monsters'. (The change from species to species is not a change involving more and more additional atomistic changes, but a complete change of the primary pattern or reaction system into a new one, which afterwards may again produce intraspecific variation by micromutation)
They called this process punctuated equilibrium. Their idea was prompted by an examination of the fossil record, which doesn't show the gradual change that it should according to the currently accepted paradigm. But their thinking was too far outside the box and it has never been accepted by the majority.
Throughout my career I have espoused thinking outside the box as an essential aid to scientific progress. It began when I was 9 years old. A new headmaster was introduced to our class at school. He gave the class a simple mathematical sum which require the conversion of so many pence into pounds, shillings and pence. One child gave an answer and the headmaster asked the class how many agreed. Everyone bar me put their hands up. I was well aware that many of the class were incapable of doing the sum; they were simply following the crowd. The master asked me my answer, which was tuppence less. He them asked me to explain my workings and as I did so I realized my error. Nevertheless, he commended me for being honest, for not being afraid of standing out from the crowd and for not being afraid of being wrong.
Something similar happened 45 years later. I was the guinea-pig in a slide session at the British Society for Haematology. We were given blood slides to interpret blind. One of the slides showed a proliferation of wild looking blast cells on a background of CLL cells. The general opinion was that this was some sort of acute leukemia, though nobody was quite sure what sort. Now I was put on the spot. I began by drawing attention to how the blast cells resembled those of infectious mononucleosus. My opinion that this was an EB virus infection in an immunodeficient patient. There was an audible drawing in of breath. People looked away in embarrassment. Then came the answer from the haematologist who had set the test. This was a reactivation of EBV in a patient with CLL after treatment with fludarabine.
I taught my students to believe their own data and not to be afraid of looking a fool.
Paradigm shifts do occur in science, and more frequently than the experts would have us believe. On a minor scale there are many examples. T-suppressor cells were once part of our immune repertoire. Then they were dismissed. You could not get a grant to look for them. Then they reappeared, though now called regulatory T cells, and they were everywhere. What happened? More data appeared.
But when a major scientific concept like global warming or Neo-Darwinian evolution is concerned the prevailing paradigm is much more robust. According to Wikpedia it is defended in the following ways:
Professional organizations give legitimacy to the paradigm.
Dynamic leaders introduce and support the paradigm.
Journals and editors write about the system of thought. They both disseminate the information essential to the paradigm and give the paradigm legitimacy.
Government agencies give credence to the paradigm.
Educators propagate the paradigm's ideas by teaching it to students.
Conferences are devoted to discussing ideas central to the paradigm.
The Media cover it.
Lay groups, or groups based around the concerns of lay persons, embrace the beliefs central to the paradigm.
Sources of funding further research on the paradigm.
Those who choose to oppose the reigning paradigm do so at their peril. They will plough a lonely furrow and must expect ridicule, abuse and oppression. They may lose their jobs or their preferment. They become scientific lepers.