Midibus

A midibus is a classification of single-decker minibuses which are generally larger than a traditional minibus but smaller than a full-size single decker and can be anywhere between and long. While used in many parts of the world, the midibus is perhaps most common in the United Kingdom, where operators have found them more economical, and to have a sufficient number of seats compared to full size single-decker buses.
Midibuses are often designed to be lightweight to save on diesel fuel (e.g. smaller wheels than on larger buses), making them not as durable as heavier 'full size' buses. Some midibuses, such as the Scania OmniTown, are heavier and therefore more durable.
In some places such as Hong Kong, some bus routes have to be served by midibuses due to the winding roads along such routes.
United States designs.
The term "midibus" is not in common use in the United States, such smaller and lighter-duty buses not being employed in public transit roles there except in some very specialized instances. For example, Muni in San Francisco operates both and versions of the Orion VII transit bus to serve routes that include some of the steeper and curvier hills.
In charter / tour roles, there is indeed a gap between the minibus (12-28 seats) and the touring coach (47-50 seats). Several shuttle bus companies such as Goshen Coach and Crystal have manufactured rear-engined vehicles that have 30–35 seats, but no generic term has ever been applied to them. They are usually lumped together with their smaller "minibus" brethren and called "minibus", or "shuttle bus". The only other alternative was to import a "short" (two-axle) version of European touring coaches, known often as "baby coaches", around long and equipped with some 30–32 seats. Examples of these include the TEMSA TS 30/TS 35 and the MCI J3500.
In the 2000s, some manufacturers introduced mid-sized bus models based on large truck frames.