We call them (or classify them as) portable structures, but I think that is mostly for tax purposes (land owners pay less (or no) tax for a 'portable' structure as opposed to permanent structure.
But you could still call them portable. A decent sized building could be loaded on a few flatbed trucks and it goes up in only a few days/weeks (provided the land prep or foundation is in place). They can be taken down, transported and re-installed, but that's not typical.