Tempo runThreshold run
A sustained effort at or near lactate threshold, run to raise that threshold.
A tempo run is a sustained effort at a comfortably hard intensity, usually around the lactate threshold, typically held for twenty to forty minutes. The classic description is the pace you could hold for about an hour in a race, run for rather less than an hour in training.
Its purpose is to raise the intensity at which lactate begins to accumulate faster than you clear it. That threshold, rather than VO2max, is often what separates endurance performances between athletes of similar aerobic ceilings.
The effort level is the part people get wrong in both directions. Run it too easy and it becomes ordinary aerobic work; run it too hard and it becomes an interval session you cannot repeat later in the week. Speaking in short phrases but not full sentences is the usual field cue.
It occupies an awkward place in polarised training models, which deliberately minimise the middle intensity in favour of mostly easy and some very hard. Threshold work still has a long track record and a clear physiological rationale, and the sensible reading is that it is a tool with a specific purpose rather than a default.
Variations exist for practical reasons. Cruise intervals, which break the tempo into several blocks with short recoveries, allow more total time at the intensity with slightly less accumulated fatigue.
Frequency matters more than heroics. One threshold session a week, held honestly at the right effort, does more than two done as time trials.
In practice, set the intensity with our heart rate zone calculator or by pace from a recent race, and treat the effort as controlled rather than maximal.