A sourdough starter that's been recently fed and is showing visible signs of fermentation -- bubbles, rising, and a pleasant tangy smell.
An active starter is what you need before mixing your dough. Look for a size increase (any amount counts), bubbles throughout the mixture, and a smell that's mildly sour but not pungent. The exact time to peak activity depends on your starter, your flour, and your kitchen temperature, but it's typically 4-8 hours after feeding.
