As used by scientists, including those who study evolution, a theory is an empirically verifiable proposition that seeks to explain some portion of reality.

Much of the requirements of theories come from the work of Karl Popper. The first requirement is that a theory be expressed in a way that can be tested. In other words, the theory must be falsifiable using data obtained during some form of observation. Theories should be expressed in the simplest possible terms. Theories that are not falsified may evolve as they are refined. According to Popper, a theory may be considered improved only when the new addition explains more than the flaw or flaws it was intended to address. This is the requirement for parsimony, and is intended to prevent theories from becoming nothing more than a series of patches, like the epicycles in Copernicus and Ptolemy's theories of cosmology, which were finally overturned by Kepler.

The common use of the word theory reduces the term to nothing more than an organized guess. But nothing can rise to the level of theory without presenting a falsifiable proposition. In fact, before a hypothesis may be considered theory, it must have already survived a series of independent experiments. In science theory is a very strong word, quite different from the common usage.