Lactate Threshold Pace
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It's the fastest pace you can sustain where lactate production and clearance are roughly in balance, beyond which lactate accumulates rapidly and fatigue sets in quickly. It typically corresponds to an effort you could hold for about an hour in a well-trained runner.

A common field test uses a 20-30 minute maximal effort time trial, then applies a percentage adjustment (often around 95-98% of that average pace or heart rate) to estimate threshold, since an all-out 20-30 minute effort runs slightly faster than true one-hour threshold pace.

A full hour maximal effort is impractical to execute accurately and pace correctly outside of racing, so a shorter, more manageable time trial gives a reliable proxy that can then be scaled down slightly to approximate true threshold pace.

It's close but not identical: threshold pace is often somewhat close to half marathon race pace for well-trained runners, while 10K pace is typically faster than threshold since it's sustained for a shorter duration. The exact relationship varies by individual fitness and event specialization.

Threshold pace is used for tempo runs and cruise intervals designed to improve your body's ability to clear lactate at faster efforts, typically run for continuous or broken efforts of 20-40 minutes total. Training right at or slightly below threshold is more effective for building this specific fitness than going all-out.