I like the American Heritiage Dictionary's definition
3.
Biologya. Change in the genetic composition of a population during successive generations, as a result of natural selection acting on the genetic variation among individuals, and resulting in the development of new species.
I believe it has all the necessary components and doesn't leave much room for obfuscation.
So let's break it down.
Change in the genetic composition of a population during successive generations
This is an essential component. We must have successive change to the DNA over generations. These changes should be passed down by reproduction. The reproductive rates of the parent generation determine the statistical saturation of these changes in the offspring generation.
as a result of natural selection acting on the genetic variation among individuals
Notice, evolution is stated as the
result of natural selection acting on the genetic variation among individuals. Evolution is a consequence of the reproductive rates of the parent generation holding these genetic variations. The reproductive rates are a consequence of the organisms' ability to reach reproductive maturity and to produce offspring while challenged by natural selective pressures. By understanding evolution as a consequence rather than a goal we can see that there is no requirement to move to increased complexity. In some cases simplicity may be more advantageous.
and resulting in the development of new species.
Of course, this is where most creationists will be held up. They will claim that small changes are incapable of developing new species.
However, the idea of a
new species is entirely based on boundaries instituted by the scientific community. Linnaean Taxonomy is largely qualitative. It is the collaboration of scientists studying these organisms which determines classification. Now, there are guidelines, but the classification of species has always been a qualitative human endeavor.
As in any qualitative classification system, there will always be some overlap and heated discussions on the deliniations within this overlap. It happens in music, it happens in science.