If "string theory" and "M-theory" are not tested yet, should they be read as "string hypothesis", "M-hypothesis"?

The question is: shouldn't M-theory be M-hypothesis, because it isn't proved yet. An implicit assumption in this question is that theory and hypothesis are mutually exclusive terms.

This is not actually how the words are used in science. Consider the case of continental drift. Continental drift was proposed by Wegener in 1912, but not generally accepted until the 1950s, by which time overwhelming evidence supporting it had accumulated. If you look in Google books in the 1930s, you will see books calling it the theory of continental drift and also saying this hypothesis. So continental drift could be both a theory and a hypothesis.

I would say scientists use theory for a coherent collection of related explanations of phenomena (relativity theory, quantum theory, theory of evolution, gene theory of inheritance), whether or not it has been proven by evidence. By this definition, string theory and M-theory count as theories.

Scientists use hypothesis for an explanation of phenomena which has not yet been proven. A hypothesis can, but need not, be an explanation of a set of phenomena that is much too narrow to qualify as a theory. So by this definition, string theory and M-theory are also hypotheses.

^{I started a comment on Peter's answer (who happens to be well placed to appreciate the context of your question) but then I found out that I had too many ideas of my own and decided it appropriate to share them here.}

I'd say that there is a lot of overlapping between both definitions (the mathematical and the scientific one).

Note for instance that in mathematics a theory, in the first sense (a self consistent set of axioms, definitions and theorems), can be studied long before it is actually applied usefully in some scientific area.

Examples abound of these cases (e.g. imaginary numbers in 16c. Italy long before electromagnetism or quantum mechanics were discovered). This is arguably the rule rather than the exception.

So that the purely mathematical sense of the word theory is not that far from the scientific acceptation of "hypotheses making predictions waiting to be proven/disproved".

Which leads to the conclusion that a theory cannot wait to be proven to deserve the rank of theory. One might even argue that it stays a theory as long as it is not disproved (after that is is just an abandoned theory ;-).

If scientists were to put theories forward only when they'd have the technical abilities to prove them, we'd be way backwards. Newton would have had to wait for telescopes to be powerful enough to spot planet Neptune and Einstein for atomic clocks to be invented.

Merriam Webster's defines theory like this:

• a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena (e.g., the wave theory of light)

• a body of theorems presenting a concise systematic view of a subject (e.g., theory of equations)

However, it also defines it like this:

• a hypothesis assumed for the sake of argument or investigation
• an unproved assumption

I think in the case of the string theory, theory is being used in the first sense of the definition. Calling it the string hypothesis would take away from what we mean to say (make the idea smaller).