The ratio between the stock of existing bitcoins and the flow created each year. The higher it is, the 'scarcer' bitcoin is by this measure: it would take X years of issuance at the current pace to reproduce the existing stock.
The ratio DOUBLES at each halving (issuance is cut in half at once), hence the staircase steps. Important: S2F is a scarcity measure, NOT a price oracle. The 'S2F model' that derived a price target from it was widely criticized and has aged poorly since 2021: do not read it as a prediction. We show it here as a monetary-scarcity indicator, to be read over time.