Niyāma: the 'fixedness of law' regarding all things; cf.tathatā - pañca-niyāma is a commentarial term, signifying the 'fivefold lawfulness' or 'natural order' that governs: 1 temperature, seasons and other physical events utu-niyāma 2 the plant life bīja-n. 3 kamma kamma-n. 4 the mind citta-n e.g. the lawful sequence of the functions of consciousness see: viññāna-kicca in the process of cognition; 5 certain events connected with the Dhamma dhamma-n e.g. the typical events occurring in the lives of the Buddhas.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.