A measure with a written definition: formula, sources, period, inclusions, level and the identity that checks it.
A metric is a computed number that two people would compute the same way from the same data. The definition is what makes it one: what is divided by what, from which export, over which period, including and excluding what, at which level, reconciling to what. Without it the number is a label, and meetings are spent on whose version is right.
A number becomes a metric when its definition is written down: formula, source of each input, period, inclusions and exclusions, the level it is computed at, and the identity that checks it.
Revenue per active account: net invoiced revenue over the trailing twelve months, divided by accounts with at least one invoice in that window, from the ledger, excluding intercompany.
A label with no definition. Two analysts produce two numbers and the meeting is about which is right.