The ratio of water to flour in your dough, expressed as a percentage using baker's math.
A dough with 500g flour and 350g water is at 70% hydration. Higher hydration (75-85%) produces a more open, airy crumb but is stickier and harder to handle. Lower hydration (60-65%) makes a stiffer dough that's easier to shape but produces a tighter crumb. Your flour's protein content determines how much water it can absorb -- higher protein flour handles more water. Start at 65-70% and adjust from there as you learn your flour.
