Medium of motion- One of the six fundamental Dravyas (substances) or five Astikāyas (extended substances);
The substance, which is the unique inevitable medium of motion, passively assisting in the motion of the Jīvas (souls or living beings) and the Pudgalas (physical substances), which are apt to undertake mo-tion; it is only one (single) in number with respect to substance; it is eternal, non-corporeal (i.e., devoid of colour etc.); it is pervading the whole Loka (cosmos); it has innumerable number of Pradeśas (the indivisible units of the substance); it is antithesis of Adharmāstikāya (fundamental substance qua medium of rest).
gamanapravṛftānāṃ jivapndgalānāṃ gatau udāsīnabhāvena ananyasahāyakaṃ dravyam dharmāstikāyaḥ.
(Jaisidī 1.4 Vṛ)
davvao ṇaṃ dhammatthikāe ege davve, khettao logappamāṇamette. kālao...sāsae... bhāvao avaṇṇe agaṃdhe arase aphāse. guṇao gamaṇaguṇe...
...asaṃkhejjā dhammatthikāyapaesā...
(Bhaga 2.125,134)
See - Adharmāstikāya.