Fundamental substance qua medium of rest -
One of the six fundamental Dravyas (substances) or five Astikāyas (extended substances); the substance which is the unique inevitable medium of rest, passively assisting in the rest of the Jīvas (souls) and Pudgalas (physical substances) which are apt to undertake rest; it is only one (single) in number with respect to substance; it is eternal with respect to time, non-corporeal (i.e. devoid of colour etc.) with respect to mode; it is pervading the whole Loka (cosmos) with respect to space; it has innumerable number of Pradeśas (indivisible units); it is antithesis of Dharmāstikāya (fundamental substance qua medium of motion).
sthānagatānāṃ jīvapudgalānāṃ sthitāvudāsīnabhāvenā' nanyasahāyakaṃ dravyamadharmāstikāyaḥ... yathā pathikānāṃ chāyā.(Jaisidī 1-5 Vṛ)
davvao ṇaṃ adhammatthikāe ege davve, khettao logappamāṇamette. kālao....sāsae. bhāvao avaṇṇe agaṃdhe arase aphāse. guṇao ṭhāṇaguṇe... evaṃ adhammatthikae vi...(Bhaga 2.126,135)
- Dharmāstikāya.