Making sourdough starter and using it in recipes is a whole other level of baking – it’s challenging at first, but once you get the hang of it, you’ll get such great satisfaction from it. That said, here are some important terms to understand before starting:A mature starter is over 14 days old and has been doubling in size and you're able to predict how long it takes a starter to reach its “peak.” Some sourdough recipes call for a starter to be used while at its peak (when it domes, is very active, and is full of air bubbles), which is about 6-8 hours after feeding. This is often also referred to as an active starter. An unfed starter is a starter that's ready to be fed again for its daily feeding/discard routine. It's past its peak and is no longer active. The bubbles are starting to deflate and the top of the starter is no longer domed.A sourdough discard is the part that has been removed before you feed the remaining starter. The discard no longer gets fed and is either thrown out or saved for recipes that call for sourdough discard.