There are two main meanings of the word theory:
- informal theory
- scientific theory
- "A coherent statement or set of statements that attempts to explain observed phenomena, which has testable implications, and which is well tested and widely accepted as true." (wiktionary)
The confusion between these two meanings is often exploited by creationists, who dismiss the scientific theory of evolution by natural selection (a well-established explanation for a number of natural phenomenon, upon which much of modern science depends) by calling it "only a theory", which for most people (especially non-scientists) evokes the idea of an "unproven conjecture" or idle speculation.