The country's main territory; the country proper. Used in contrast to vassal states, outlying islands, etc.
I know about the different kinds of usages for ～に対して (対象、対比、割合) and its various forms, but this sentence kind of threw me for a loop. From the description, it is obvious that it must be the usage of 対比 here, contrasting the 'mainland' with 'vassal states' and 'outlying islands'. But I've never seen the 対比 usage used in the form of ～に対して～ but rather N＋であるのに対して／N＋なの対して. Is the form as it is given in the dictionary entry above a commonly used one and if yes, one could only guess from the context itself which usage is intended, right?