Generally, evolution is any process of change over time. In the context of life science, evolution is a change in the traits of living organisms over generations, including the emergence of new species. Since the development of modern genetics in the 1940s, evolution has been defined more specifically as a change in the frequency of alleles in a population from one generation to the next.
Because the term evolution can be used in many different contexts, even within biological circles, it is useful to correctly identify some of the key terms. Evolution, strictly speaking, is the change in frequency of genetic occurrences within a given gene pool over time. The theory of evolution is the scientific model that describes the descent of all living organisms from a common ancestor. Natural Selection is the principal mechanism that causes evolution. In common parlance the word "evolution" is often used as a shorthand for both the modern theory that all extant species share a common ancestor as well as the mechanisms through which natural selection acts to change populations over time.
As the theory has become widely accepted in the mainstream scientific community, it has replaced other explanations including creationism and Lamarckism. Skeptics often Creationists sometimes minimize the explanatory power and validity of evolution theory by criticizing it as "just a theory" using "theory" as synonymous with "conjecture" or "speculation", instead of the technical, scientifically accepted use of the word "theory" to mean a model of the world (or some portion of it) from which falsifiablehypotheses can be generated and be verified through empirical observation. In this sense, evolution is a very powerful theory.