Velocity stability
How consistent a team's completed work is from sprint to sprint, a stable velocity makes forecasting reliable, while a volatile one makes it guesswork. Stability matters more than the absolute number: a team averaging 30 points plus or minus 3 forecasts better than one swinging between 20 and 50, and chasing a higher average often destabilises the very predictability that made velocity useful.
Velocity's value is almost entirely in its stability, not its absolute number. A team delivering a steady 30 points a sprint is far more plannable than one bouncing between 20 and 50, even if the bouncy team's average is higher. Stability is the signal that estimation and capacity are calibrated; volatility says something upstream is noisy, scope, interruptions, or inconsistent sizing. This is also why comparing velocity across teams is meaningless: the unit isn't shared.