W re to this as used by Buddhists, it is actually related to how Gatama was positioned metaphysically rather than, as often commonly thought, moderate choices between conventional social extremes. Tho this is evan taught in many Buddhist circles. (If we investigate his life and advice we can hardly call him a social middle of the roader. Furthermore if this was refering to a midddle ground between indulgence and ascetisism the position would not be unique, also it would be err to reletiveness. Thus improper and unworthy of using as a description of his teachings).
In his era as in ours there were two strong positions on exisitence, eternalism and nihilism.
He debated against these and equated both with being incorrect views on life.
With eternalism the belief is that there is a seperate entity connected with each body form and it exists and travels from form to form, body to body, as each decays ect ect, eternally. The egoic dream that we dont really die, ever.
Nihilism simple put might be considered the opposite. We live, we die, thats all. We may be familiar with the group of the same name in europe from the last cent. who promoted a way of "anything goes living" because in their view of things there is no actual lasting reality to any of it.
Tho we can certainly understand how these viewpoints can represent stages of understanding, appropriate and evidenced, the primitive moving on to the more evolved, still this Buddha strongly stated that both of these understandings were errroneous and would only delay enlightenment and ultimately only continue to perpetuate suffering.
His position was termed annata, sometimes called no-self, but since we all know something as a self ( and he also refered to reincarnation) then the quandry is answered in a most particular way. Evan the term nirvana has a few different defintions coming from so long ago with all the linguistic and idiomatic understandings needed to really get a grip on the posibilities of meaning.( see Thanissaro's work "The Mind Like Fire Unbound" on how nirvana and annata have been missconstrued over the ages)
Gautama, the Buddha, was reported to have rarely commented on such metaphysical topics and rather that his silence was part of a comment on the worthyness of the questioner and whether the question was being asked for the right reason.
Still it is reported that he did reveal truths re these matters and that he held distinct views on such things and thus his particular teaching at his time was known as the middle way as opposed to the eternalist Jain leader of the time, Mahavira, and other religious leaders whose phiosophies were rooted in nihilism.
Still all of this is debated and analysed, used and missused by us. Thus strengthening the position of teachers who remain silent and allow the motivation of the questor, not earlier than ready, to find and really know the truth without other reliance.