Sour is a mother that has been refreshed with flour and water. Or, yet another term seemingly made up to confuse the beginning sourdough baker. It's just an active sourdough starter.
A bit more discussion of starters and sponges. First, there are some equivalencies to be aware of:
Mother = chef - it only depends on the consistency - a chef is dough-like, while a mother is batter-like. Most people in the USA ignore the subtleties and just call this starter.
Sour = levain - again it depends on the consistency of the starter - a Sour, batter-like; a levain, dough-like. The difference between these terms and the ones above is that they represent the term that indicates that the starter is activated.
Chef, levain, biga naturale, mother, and sour contain only natural yeast cultures.
All of the above - biga, biga naturale, chef, levain, mother, poolish, and sour are often referred to as either starters or sponges.