A scientific theory is the expanation of observed phenomena. Observation and experimentation provide facts, the scientist considers these facts and postulates a theory. The theory then suggests further experiments, which will confirm/alter/deny the theory. Refined Theorising and refined experimentation enable the scientist to discover more facts and improve the thoery further.  

Theories begin as educated guesswork, but become very sophisticated and accurate. The theory of evolution has been modified throughout the last 150 years (not least by the decoding of DNA) It is such an accurate model that it often predicts the existence of unknown fossils, which are later discovered.

You can improve the theory of evolution, but you can't disprove it anymore. It is simply too accurate. The theory of evolution was a sophisticated theory before DNA was understood. Then Crick and Watson cracked the DNA roadmap and completely confirmed everything evolutionary scientists had theorised. The theory of evolution is interlaced with 100,000s of repeatable observations and experiments. The facts derived from these experiments confirm the theory again and again and again. You can prove evolutionary theory.

Creationist theory, on the other hand, is ridiculously easy to disprove. It fails to conform to ANY of the observable phenomena. Only non-scientists consider the two theories have mutual worth. Creationist theory is not a scientific theory.

dancer7, as you can see from my post, I cannot agree with statements such as yours 'You can prove evolutionary theory'. That is going beyond the acceptable.

What do scientists mean when they use the word 'theory'?

You often hear non-scientific people say, "The theory of evolution is only a theory." And they imply that the word 'theory' means 'best guess' or 'sketchy idea'.

But what do scientists actually mean by a 'scientific theory'. Is it different from the public's understanding of the word. Can you prove a theory?

Dictionaries are very helpful in these discussions. I have picked the following, highlighting what I think is most helpful with your questions:

[from Wordweb]

A well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena

[Merriam-Webster]

a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena

[Dictionary.com]

a coherent group of general propositions used as principles of explanation for a class of phenomena

The problem stems from the other uses of the word theory, also present in the same dictionaries. What you see above are those that, in the minds of scientists, would apply to the theory of Evolution. It is a totally different status from other uses of the word, some of them more akin to hypothesis, conjecture or speculation. This prompts the formulation of differential statements, such as the following in Dictionary.com:

Theory, hypothesis are used in non-technical contexts to mean an untested idea or opinion. A theory in technical use is a more or less verified or established explanation accounting for known facts or phenomena: the theory of relativity. A hypothesis is a conjecture put forth as a possible explanation of phenomena or relations, which serves as a basis of argument or experimentation to reach the truth: This idea is only a hypothesis.

Finally, when Pope John Paul II made his famous statement to the effect of 'Evolution is more than just a hypothesis', he added 'there is more than one theory'. Both statements must be acknowledged as correct, the second however with a qualification: the diversity of scientific discourses on Evolution deal with details, such as accepting or not Kimura's neutral theory, or Eldredge & Gould's punctuated equilibria theory (I, for one, accept both as very illuminating complements of the work of Charles Darwin). The principles laid down by Charles Darwin, complemented by the Genetics amendments made by Fisher, Wright and Haldane in the early 20th century, are taken as the best scientific explanation on the diversity of extant and fossil life. Even when creationists and ignorant journalists construe the Flores man and Ardi as evidence against the Evolution theory. Quite the contrary, they serve as new examples of the original way of explaining evolution set by Charles Darwin.

Not proof, but examples. A theory as defined above is an explanation, not a fact, hence no proof required. But while you cannot deny facts, you can deny theories. Saying that a theory is verified (like in one of the quotes above) it means that it has not been denied, actually.

This is why I choose to write 'best scientific explanation'. The challenges of new facts are a constant test on the theory, and scientists would be the first to acknowledge its invalidity. Science cannot afford to be dogmatic.

I am not a scientist; however, my understanding of the term "theory" as it applies to scientific principles is that it is a proposition that has never been proven, but it has also not been disproven and there is substantial evidence collected over time to suggest the strong likelihood that it is in fact proveable.

A scientific theory by definition cannot be proved beyond doubt, for if it is, it then moves into the realm of scientific fact. Scientific theories, however, are far more than unsubstantiated guesses or suppositions. They develop through extensive research and the accumulation of scientific data that support their soundness. The theory of evolution, for example, is supported by physical evidence and the application of scientific technology, such as carbon dating.

I think that the use of the word "theory" in the scientific community versus the general population might be different because of its level of precision in the former, as opposed to the latter.  In the scientific community, when one uses the word "theory" it is used to describe something more than a vague supposition.  Even the most "untested" of theories have grounding in some observable data and logic.  The vague nature of the word increases when it is extrapolated to the outside world, where one sees a great deal of "sketchiness" in the use of the word theory.