A mid-level dev has more experience with his or her main programming language, normally it is normal to have fluency with different programming languages (more than 1).
They have enough experience to maintain a small project or infrastructure in the production level that is conventional nor critical.
They know to plan ahead and estimate how long they are going to be coding a solution. (with an error margin that is not higher than 20%).
The concept of software quality and quality control is already well understood. They know how to revise other devs programs and check the normal workflow of the company.
Usually, they have 4 to 5 years of experience.
They start to understand that the code is only a small part in the world and are able to better understand clients, end-users and are becoming to improve in their sales skills. Sometimes, they can have a hard time when they need to change some requirements or find alternatives solutions.
They have already understood the notion of scaling systems and are able to analyze their colleagues, not knowing yet to map their complete profile.
At this point, we attend a dev that has at least a B1 (EU) level in any foreign language.