What constitutes the "missing middle"?

[Surrealplaces]

I feel there's a pretty clear definition for 'middle' type housing - 'middle' being anything more dense than SFH, up to any type of structure that has direct access from the outside. For me a 3 story building where the majority of units are accessed from the inside of the structure is an apartment plain and simple. At this point there is no difference between say that or a high-rise, at least from a definition perspective. Other intangibles make the two different.

With that type of definition, it then becomes a case of whether it's 'missing' or not, and what level or percentage would require it to be 'missing'?