Because ideas about evolution by natural selection have never been proven false, what term can be used to describe evolution by natural selection?

Technically, a scientific theory can never be irrefutably proven. Observations that corroborate a theory and increase its credence, but can never prove it. Contradictory evidence can confute a theory, and if such a case happens, then the theory will have to be modified or replaced so that it accounts for the evidence.

The meaning of theory in a scientific sense is a little different than what is used in the common vernacular. While theory to a layman may mean little more than speculation, in scientific terminology, a theory is an explanation of the natural world that can be used in a predictive capacity and thus tested rigorously and repeatedly.

Evolution will ever remain a theory, as any theory that can be considered scientific has to be testable and refutable. However, as science has matured in the "down to earth" fields, I do not think that evolutionary theory will ever be disproven in whole, though it might be modified to account for new discoveries. These modifications will probably be very specific modifications and the general theory would still stand.