What is the biggest difference between junior and mid-level UI designers?

What is the biggest difference between junior and Mid-level UI designers?
How long junior is a junior and what skills, knowledge are needed to be considered mid-level?

Junior designers sometimes lack the experience to look beyond the parameters of the task they are set. In contrast, senior designers have the deeper knowledge and experience necessary to challenge and, where appropriate, to work beyond the brief to produce a design with greater functionality or improved user flow.