Astikaya is a Jaina philosophical term meaning ‘substance having extension’. Jainism classifies substance under two main categories, jiva and ajiva; the former is conscious while the latter is non-conscious. The entire universe is brought under these two uncreated, eternal, con-existing categories. Ajiva includes matter, space, time, rest and motion. The conscious jiva (spirit), pudgala (matter), akasha (space), dharma (motion) and adharma (rest) are described as astikaya dravyas or substances which possess constituent parts having extension, while kala (time) is characterized as anastikaya Dravya, not having the quality of extension.
Astikaya substances have extension but this term, as used in
Jainism, is not merely in the sense of material extension. As these substances
exist, they are termed asti and because they have many pradeshas they are
termed kayas. While the term Pradesha is used to refer to that part of space
occupied by an atom of matter, such pradeshas contain not only matter but particles
of other substances also. Thus each substance as set out by Jainism has its own
pradesha. That which has more than one pradesha, where they are countable,
innumerable or indefinite, belongs to the asitkaya category.