Seeking a clear definition of shastra -

I'm looking for a clear definition of zAstra.

Haridas Dasji offers three definitions in his Abhidhan:

zAstra [ zAsu + STran ]
1) (hari 5|364) veda, tantra o purANAdi | (Vedas, Tantras and Puranas.)
2) (hari 3|99) anuzAsana | (Instructions, commandments.)
3) (daza 29) vidhi-vAkya | (Delineations of rules.)

Hari here refers to harinAmAmRta-vyAkaraNam, daza refers to daza-zlokI-bhASyam.

We frequently hear the phrases zruti-zAstra and smRti-zAstra, the former referring to Veda, Upanishads and Brahma-sutra, the latter referring to Puranas, Itihasas and so forth.

What I'm wondering is whether there's a clear definition limiting the term to certain texts or certain genres of texts, or whether the term is in fact rather ambiguous.

The first issue we run into in the Gaudiya context is the concept of guru, sAdhu and zAstra. Where is the line drawn between sAdhu and zAstra? Are the writings of Rupa Goswami considered zAstra? Yes, I would certainly think so. Are the writings of Narottama Das Thakur considered zAstra? According to Visvanatha Chakravarti (Stavamrita-lahari Narottama-prabhor-ashtakam 7), prAmANyam eva zrutivad yadIyam, his words are as authoritative as the zruti. Then, are the writings of Visvanath Chakravarti considered zAstra? And what of the writings of a contemporary sAdhu, are his writings considered zAstra? If not, will they eventually be considered zAstra? If so, why?

Does zAstra always refer to works of divine authority? If so, why is it applied as a suffix to varieties of dharma-zAstra, artha-zAstra and even kAma-zAstra written over the millennia? If the Puranas are considered zAstra, why then is one of them considered pramANam amalam, implying that others are less than that? The eighteen great Puranas are divided into three groups of six, one for each mode of nature, containing contrary conclusions on the supremacy of Vishnu along with a host of other contradictory statements. Yet we often refer to zAstra-pramANa.

Some traditions of Vedanta refer to praSThAna-traya, or the Upanishads, the Brahma-sutra and the Bhagavad-gita. Yet I do not know whether they would also consider other texts zAstra, and I am also mystified over the origin of this three-fold division, in particular over the inclusion of the Bhagavad-gita originating as a section amidst Mahabharata, a text classified among the Itihasa and thus on par with the Puranas.

Does anyone have any further thoughts on this?

About limiting the shastras to pramAnam amalam and so, there is this wise verse from Canakya Pandit - ananta shastram bahulas ca vidya svalpas ca kalo bahu vighnata ca - "Scriptures are unlimited and there are so many sciences. Time is short and we face so many obstacles." yat sara bhutam tad upasaniyam hamsah yathaiva ksiram ambu madhyAt - "So the essence (of all this) is worshipable, just as the swan filters the water out of the milk." And so we come to the authority of shastra, again using the process of distillation - nana shastra vicaranaika nipunau saddharma samsthapakau - "The Gosvamis established the true religion because they were expert in considering various scriptures (from which they distilled the essence)."
The question of Visvanatha's books being shastra is interesting. Back in the early 90s I heard that the babajis consider VC a sadhana siddha while they consider the aforementioned older Acaryas as nitya siddhas. Of course, Krsnadeva Sarvabhauma proclaims VC to be an incarnation of Rupa Gosvami, but that may be seen as his personal Guru-bhakti......

excellent question. I have been thinking about it for a very long time (as in "decades")

one of the risks I see quite often, is that some sadhus present very narrow interpretation of devotional life and it's practices and turning their words into shastra seems to legitimize such narrow views as authoritative and final, thus "cheapening" the very concept of shastra.