A theory has its genesis in a hypothesis, which is a working assumption as to why we observe something—an educated guess. To test this assumption, scientists conduct experiments that either disprove or correlate with the hypothesis. Over time, if a hypothesis continues to stand up to scrutiny and many different experiments, the scientific community may begin referring to it as a "theory." In essence, this means that because the hypothesis has not been disproved over many years and no other known hypothesis works, then we can be reasonably sure that it’s accurate.
Theories, however, are not imperishable. If new technology allows better experimentation, for example, a theory may need to be discarded.
Where Evolution Falls Short
Two problems prevent anyone from legitimately calling evolution a theory. First, there's no direct, observable experiment that can ever be performed. Scientists can measure bones, study mutations, decode DNA, and notice similarities in morphology (the form and structure of animals and plants), but they can never test evolutionary events in the past.