A key performance indicator (KPI) is a specific, quantifiable metric selected to measure progress toward a defined business objective, chosen deliberately from the broader set of available data because it most directly reflects success or failure for that goal.
The distinction between a KPI and a regular metric is intent, not the number itself — pageviews might be tracked by every site, but only a business that has decided traffic growth is a primary goal would treat pageviews as a KPI. Effective KPIs are usually described as being aligned to a specific, time-bound goal rather than being a vague, all-purpose 'health' metric, since a KPI that doesn't clearly indicate progress toward something specific tends to lose its usefulness as a decision-making tool.
A subscription business focused on growth might set monthly recurring revenue as its primary KPI, while a content publisher focused on audience size might instead track monthly unique visitors — the same analytics platform can supply both, but each business elevates a different number to KPI status based on its own goal.
Every KPI is a metric, but not every metric is a KPI — a KPI is specifically a metric a team has chosen to represent progress toward a defined goal, while most other metrics are simply available data that may or may not inform decisions.
Most guidance favors a small, focused set, often cited as around three to five per goal, since tracking too many KPIs at once tends to dilute focus and make it unclear which numbers actually matter most.
Yes — as business priorities shift, the metrics chosen as KPIs typically shift with them; a KPI is tied to a specific goal or stage of the business, not a permanent, fixed choice.