Like God, the word evolution has unfortunately brought untold suffering and confusion to the human race, but it need not be this way.
At the most basic level the definition of the word evolution is change. The American Heritage Dictionary more specifically defines evolution as, “…a gradual process in which something changes into a different and usually more complex or better form.” That's pretty general, perhaps too general, so in scientific, religious or philosophical discussions like the ones we have here, I define evolution as the idea that all organisms on Earth derived sequentially from a LUCA (lowest universal common ancestor) in a process that produced descendants who differed morphologically or physiologically from their ancestors. Use of the word evolution can also refer to the historical development of a group of organisms such as the evolution of insects.
On November 24th, 1859 Charles Darwin’s On the Origin of Species by Means of Natural Selection made its debut. Thomas Malthus' Population, combined with observations made while sailing on the H.M.S. Beagle influenced Darwin's theory of evolution by descent with modification via natural selection. In Origin we find two related but quite distinct meanings of evolution: special and general theory. Loosely defined, Darwin’s special theory refers to the tendency of an organism or group of organisms to adapt in response to changes in their environments, and microevolution, variation, mutation, adaptation and natural selection are all related, and in some cases, synonymous terms. Using the classic example of the finches, geographical isolation of a species can prevent interbreeding with the parent population so that a series of minor variations may eventually result in an entirely new daughter population or subspecies. Biologists, writers and other scientists have referred to such changes as microevolution.
Darwin’s general theory is the extrapolation of microevolutionary principle to explain the full sum of living organisms and postulated that all life came from a lowest universal common ancestor, and is sometimes referred to as macroevolution, but don't let these seemingly innocent prefixes fool you, as they also afford plenty of misery to intelligent discussions about evolution (see the related posts at the bottom for examples).
The theory of evolution itself has also evolved, and continues to evolve as scientific knowledge continues to grow and self-correct. Darwin didn't have the knowledge of genetics he needed to really complete his puzzle, but his ideas of natural selection with genetic mutation as described by Gregor Mendel (geneticist) became known as the modern synthesis. Theodosius Dobzhansky (geneticist), Julian Huxley (zoologist), Ernst Mayr (ornithologist), Bernhard Rensch (zoologist), George Gaylord Simpson (vertebrate paleontologist) and George Ledyard Stebbins (botanist) were among those who called the ideas the Synthetic Theory of Evolution, which interpreted evolution as the results of gene flow, genetic drift, mutation and natural selection. By the early 1970's, Stephen Jay Gould (paleontologist / evolutionary biologist) and Niles Eldredge (paleontologist) focused on concepts of stasis and cladogenesis, and their ideas are sometimes referred to as punctuated equilibrium.
Steven M. Stanley (paleontologist / evolutionary biologist) has noted the introduction of entirely new species of Polynesian butterflies exclusive to Hawaiian banana trees. His documentation of the devil’s pupfish, found in the 92-degree springs of Death Valley exclusively, represents what appears to be an entirely new species that has appeared on the scene in the last 10-30,000 years. I'm no biologist, but I've read that under such harsh conditions, bacteria can create new proteins and metabolic pathways, which is something a punctuated evolutionary sequence would anticipate.