In regions where double-deckers are not common, the term single-decker may lack common usage, as in one sense all other main types of bus have a single deck. Also, the term may become synonymous with the name transit bus or related terms, which can correctly be applied to double-deckers too.
With their origins in van chassis, minibuses are not usually considered single-deckers, although modern minibus designs blur this distinction. Midibuses can also be regarded as both included with and separate from standard single-deckers, in terms of full size length and vehicle weights, although again design developments have seen this distinction blurred. Some coach style buses that do not have underfloor luggage space can also be correctly termed as single-deckers, with some sharing standard bus chassis designs, such as the Volvo B10M, with a different body style applied.Source: Wikipedia Single Decker Article