The defining characteristic of biglaw is high stakes and high values invoked, which almost always means corporate - though some white collar, investment, and T&E work sneaks in for ultra high networth individuals. Various things like large size and ‘prestige’ all trickle down from the kernel of high stakes / high value legal work.
“mid law” is thus whatever exists a step down. Corporate law for smaller corporations with lower stakes, private law for lower networth individuals, etc. There’s no bright line as to where it exists.
I think that is what people think of when they mean midlaw- regional biglaw. Firms in secondary/tertiary markets aren’t as big and don’t have as high profile clients, but many pay “market” for that location.
It doesn’t matter. The term only has meaning in broad strokes. Most people would consider just about any entry level / SA arrangement with the promise of a 6-figure biglaw though.
I generally think “mid-law” is basically regional biglaw. Start at usually between 70-110k (depending on the market), firms either hire through SA’s or post-clerkship (mostly state clerkships, though they probably pull some D.Ct. people as well who prefer to stay in market). Size doesn’t really matter; some firms are bigger than others.