'mental development' (lit. 'calling into existence, producing') is what in English is generally but rather vaguely called 'meditation'. One has to distinguish 2 kinds: development of tranquillity (samatha-bha^vana^), i.e. concentration (sama^dhi), and development of insight (vipassana^-bha^vana^), i.e. wisdom (pan~n~a^).
These two important terms, tranquillity and insight (s. samatha-vipassana^), are very often met with and explained in the Sutta, as well as in the Abhidhamma.
Tranquillity (samatha) is the concentrated, unshaken, peaceful, and therefore undefiled state of mind, whilst insight (vipassana^) is the intuitive insight into the impermanence, misery and impersonality (anicca, dukkha, anatta^; s. tilakkhana) of all bodily and mental phenomena of existence, included in the 5 groups of existence, namely, corporeality, feeling, perception, mental formations and consciousness; s. khandha.
Tranquillity, or concentration of mind, according to Sankhepavannana (Commentary to Abhidhammattha-sangaha), bestows a threefold blessing: favourable rebirth, present happy life, and purity of mind which is the condition of insight. Concentration (sama^dhi) is the indispensable foundation and precondition of insight by purifying the mind from the 5 mental defilements or hindrances (ni^varana, q.v.), whilst insight (vipassana^) produces the 4 supra mundane stages of holiness and deliverance of mind. The Buddha therefore says: "May you develop mental concentration, o monks; for who is mentally concentrated, sees things according to reality" (S. XXII, 5). And in Mil. it is said: "Just as when a lighted lamp is brought into a dark chamber, the lamp-light Will destroy the darkness and produce and spread the light, just so will insight, once arisen, destroy the darkness of ignorance and produce the light of knowledge."
Vis.M. III-XI gives full directions how to attain full concentration and the absorptions (jha^na, q.v.) by means of the following 40 meditation subjects (kammattha^na):
10 kasina-exercises (s. kasina). These produce the 4 absorptions
10 loathsome subjects (asubha, q.v.). These produce the 1st absorption.
10 recollections (anussati, q.v.): of the Buddha (buddha^nussati), the Doctrine (dhamma^nussati), the Brotherhood of the Noble Ones (sangha^nussati), morality, liberality, the heavenly beings, death (maranasati, q.v. ), the body (ka^yagata^sati, q.v.), in-and-outbreathing (a^na^pa^na-sati, q.v.) and peace (upasama^nussati, q.v.). Among these, the recollection (or mindfulness) of in-and-out breathing may produce all the 4 absorptions, that of the body the 1st absorption, the rest only neighbourhood-concentration (upaca^ra-sama^dhi, s. sama^dhi).
4 sublime abodes (brahma-viha^ra, q.v.): loving-kindness, compassion, altruistic joy, equanimity (metta^, karuna^, mudita^, upekkha^). Of these, the first 3 exercises may produce 3 absorptions, the last one the 4th absorption only.
4 immaterial spheres (arúpa^yatana, s. jha^na): of unbounded space, unbounded consciousness, nothingness, neither-perception-nor-non-perception. These are based upon the 4th absorption.
1 perception of the loathsomeness of food (a^ha^re patikkúla-san~n~a^), which may produce neighbourhood-concentration
1 analysis of the 4 elements (catudha^tu-vavattha^na, s. dha^tu-vavattha^na), which may produce neighbourhood-concentration.
Mental development forms one of the 3 kinds of meritorious action (pun~n~a-kiriya-vatthu, q.v.). 'Delight in meditation' (bha^vana^-ra^mata^) is one of the noble usages (ariya-vamsa, q.v.) .