Lactate threshold
The intensity above which lactate accumulates faster than the body can clear it.
The lactate threshold is the exercise intensity at which blood lactate starts to climb steeply, because your muscles are producing it faster than the rest of the body can clear it. Below it you are in a steady state you could hold for a long time; above it a clock starts running.
It matters because among trained athletes it predicts endurance performance better than VO2max does. Two runners with the same maximum oxygen uptake can be minutes apart over ten kilometres, and the one who can hold a higher fraction of that maximum without accumulating lactate is the one who wins.
Lactate itself is not the villain of the story. It is a fuel that muscles and the heart use, and it is not what makes you sore the next day. It rises alongside the metabolic conditions that limit hard exercise, which makes it an excellent marker and a poor culprit.
The terminology is genuinely messy, and it is worth knowing that before reading anything else about it. A major review of lactate threshold concepts catalogued a long list of competing definitions, thresholds and calculation methods, all called much the same thing and none of them interchangeable. When two sources disagree about your threshold, they are often measuring different things.
You do not need a lab to work with it. The intensity you could sustain for roughly an hour, the fastest pace at which you can still speak in short phrases, and a heart rate band derived from a maximum estimate all land close enough to guide training. A lactate meter adds precision that matters at the elite end and rarely below it.
It responds well to training, and to both ends of the intensity range: a large base of easy aerobic volume improves clearance capacity, while tempo and threshold work raise the intensity at which the crossover happens.
In practice, use our heart rate zone calculator to set the bands and the running pace calculator to hold the pace, and judge progress by the speed you can sustain at the same effort rather than by any single number.