To me there's some marketing going on here. A studio potter is a Leach or Robineau style potter. The potter makes the pots, decorates the pots, glazes the pots, fires the pots, and markets/sells the pots. There is a requirement, in my mind, that studio potters are also producing high-quality work suitable for a gallery or commission. Studio pottery is a label for marketing ceramics to distinguish it from ceramics made in a factory, and distinguish it from the now practically extinct country pottery. So, studio pottery is pottery marketed as art (including everyday art) by the person who made the pottery alone or with assistance from apprentices.
Why am I focused on market? Compare Chris' definition to the 19th cent. southern potter making crocks and jugs, or someone like Isaac Button. Those people marketed their ware to country people for everyday use, but they also did all the work themselves in shops and buildings that they owned. That's not studio pottery, even though its made under similar conditions as studio pottery.