Everyone has their own definitions. Here are mine. Do note that a lot of people who think that they are senior, actually aren't by mine, and senior is something that can be reached by some with very little experience.
Junior - Needs close supervision. Can't reliably develop something more than 1k lines.
Mid level - Has learned to follow a design consistently. Doesn't keep an eye on the bigger picture. Likely to fall apart around 10k lines.
Senior - Can work in the weeds and keep an eye on the business as a whole at the same time. In my experience is likely to be able to design something that can scale to 100+k lines.
Junior - Needs close supervision. Can't reliably develop something more than 1k lines.
Mid level - Has learned to follow a design consistently. Doesn't keep an eye on the bigger picture. Likely to fall apart around 10k lines.
Senior - Can work in the weeds and keep an eye on the business as a whole at the same time. In my experience is likely to be able to design something that can scale to 100+k lines.