स्य

Derived from स (sá) with a suffix य (yá)

स्य • (syá) m

1. (masculine) that
   • c. 1700 BCE – 1200 BCE, Ṛgveda 1.52.1:

     त्यं सु मेषं महया स्वर्विदं शतं यस्य सुभ्वः साकमीरते।
     अत्यं न वाजं हवनस्यदं रथमेन्द्रं ववृत्यामवसे सुवृक्तिभिः॥

     tyaṃ su meṣaṃ mahayā svarvidaṃ śataṃ yasya subhvaḥ sākamīrate.
     atyaṃ na vājaṃ havanasyadaṃ rathamendraṃ vavṛtyāmavase suvṛktibhiḥ.

     I GLORIFY that Ram who finds the light of heaven, whose hundred nobly-natured ones go forth with him.
     With hymns may I turn hither Indra to mine aid,—the Car which like a strong steed hasteth to the call.

Unlike तद् it is used adjectivally only, hardly ever occurring without its substantive. It never begins a sentence expect when followed by particles उ, चिद्, नु, सु.