The aerobic threshold is the fastest pace you can run indefinitely. Or at least it FEELS like you can run it indefinitely. If you're well-conditioned, you should be able to run for an hour or more at aerobic threshold without it becoming strenuous. Now, aerobic threshold can't be maintained forever because of two reasons, namely, your legs get tired and you run out of glycogen, but until this starts to happen, you ought to feel unstoppable. The aerobic and anaerobic thresholds are FEELINGS, and you need to learn what they feel like to you.
Put simply, aerobic threshold is the fastest you can run without trying at all. Anaerobic threshold is the fastest you can run without trying very much.
Again, it's a feeling, not a real pace writ in stone. That's why most people negative-split a good tempo run--their body is getting more warmed up and comfortable, thus becoming more efficient throughout the run.