sikkhā: the 'training', which the Buddha's disciple has to undergo, is 3-fold: training in higher morality (adhisīla-sikkhā), in higher mentality (adhicitta-sikkhā), and in higher wisdom (adhipaññā-sikkhā). This 3-fold training refers to the 3-fold division of the the 8-fold Path (magga, q.v.) in morality, concentration and wisdom (sīla, samādhi, paññā). In D. 16 and A.IV,1 it is said:

"It is through not understanding, not penetrating noble morality ... noble concentration ... noble wisdom ... noble deliverance that I, as well as you, have had for such a long time to pass through this round of rebirths.''

"This then is morality, this concentration, this wisdom, this deliverance. Being endowed with morality, concentration brings high fruit and blessing. Being endowed with concentration, wisdom hrings high fruit and blessing. Being endowed with wisdom, the mind becomes freed from all cankers (āsava q.v.) namely, from the sensuous canker (kāmāsava), from the canker of existence (bhavāsava) from the canker of opinions (diṭṭhisava) from the canker of ignorance (avijjāsava).

Buddhist Dictionary, Manual of Buddhist Terms and Doctrines, by NYANATILOKA MAHATHERA

sikkhā : [f.] study; discipline.

A.P. Buddhadatta Mahathera, Concise Pali-English and English-Pali Dictionary [available as digital version from Metta Net, Sri Lanka]