Holding a muscle at length without moving, typically for 30 to 60 seconds, to increase range of motion over time.
It works for flexibility. Held 30 to 60 seconds, three to five times a week, it produces real range-of-motion gains within about four weeks, mostly by raising tolerance to stretch rather than by lengthening tissue.
The old warning that it ruins performance is overstated. Effects appear after long holds of 60 seconds or more and are small and short-lived; a 20-second hold before training costs nothing measurable. The practical rule is still to keep long holds after training or in their own session, and warm up with dynamic work instead.
Program it where a client actually lacks range, tested rather than assumed. Stretching a shoulder that already reaches 180 degrees passively is time spent on the wrong problem.