Phil Johnson does an excellent job here. His definition is as clear and concise as any that I have seen.
However, I do think I have come up with a one sentence, pithy, easily remembered definition:
Truth without dogma, and, dogma without truth.
Emergents will tell you that they do believe in truth with the qualification that they might be wrong. On the other hand, they're absolutely sure you're wrong about the truth that you're absolutely sure is true...