The single metric that best captures the core value your product delivers to customers, picked so that moving it up forces the rest of the business to follow.
Through the early 2010s Facebook's growth team treated DAU as the metric that mattered above all else, with revenue and ads treated as downstream. Every team's roadmap had to explain its line of effect to DAU, which is why feature work that didn't move it kept getting killed even when it looked promising in isolation.