PZ Meyers asked his readers to try to supply a working definition of evolution in one sentence. Here was his initial suggestion:

Evolution is a well-confirmed process of biological change that produces diversity and coherent functionality by a variety of natural mechanisms.

This actually doesn't seem very good. "Well-confirmed" is unnecessary, and as one of the comments there pointed out, defensive. Evolution isn't necessarily "biological". It doesn't always increase diversity or coherent functionality, and "natural" is another poor choice of words (is artificial selection a natural mechanism?).

Larry Moran objected to Meyers leaving out permanent genetic changes and the fact that evolution works on populations, so Meyers amended his definition to this:

Evolution is a well-confirmed process of biological change that produces heritable diversity and coherent functionality in populations by a variety of natural mechanisms.

Still sounds bad to me.

Someone else noted Richard Dawkins' attempt at a one-sentence description of evolution:

Life results from the non-random survival of randomly varying replicators.

That's pretty good, but survival is not always non-random (shit happens).

Here's my offering:

Evolution is the change in the distribution of heritable traits in a population over time.

This definition doesn't limit the concept of evolution to biological populations. It doesn't entail an increase or decrease in variability or functionality or complexity (evolution can lead to increases or decreases in all those things). This definition doesn't even necessarily entail a genotype/phenotype dichotomy (theoretically you don't need to have the phenotype encoded in a secondary structure and transcribed each generation...a phenotype could just directly spawn another phenotype with shared characteristics, some of which are altered through reproductive operators like mutation).

Of course, my definition doesn't specify the mechanisms by which the change in the distribution of heritable traits takes place (such as various types of selection, genetic drift, etc.), but that's a bit beyond the scope of a single sentence (unless you want to make it really long).

Anyway, if you've got issues with my definition, quibble away, and/or provide your own.