Established upon moral discipline, the disciple takes up the practice of meditation, intended to stabilize the mind and clear away the obstacles to the unfolding of wisdom. Because meditation elevates the mind beyond its normal level, this phase of practice is called the training in the higher mind (adhicittasikkhā). Because it brings inner stillness and quietude, it is also called the development of serenity (samathabhāvanā). Successful practice results in deep concentration or mental unification (samādhi), also known as internal serenity of mind (ajjhattaṃ cetosamatha). The most eminent types of concentration recognized in the Nikāyas are the four jhānas, which constitute right concentration (sammā samādhi) of the Noble Eightfold Path. Beyond the jhānas lie the four formless attainments (arūpasamāpatti), which carry the process of mental unification to still subtler levels.
Discourses from the Pali Canon, p. 257