The term subitism points to sudden enlightenment, the idea that insight is attained all at once. The opposite approach, that enlightenment can be achieved only step by step, through an arduous practice, is called gradualism.
The application of the term to Buddhism is derived from the French illumination subite (sudden awakening), contrasting with 'illumination graduelle' (gradual awakening). It gained currency in this use in English from the work of sinologist Paul Demiéville. His 1947 work 'Mirror of the Mind' was widely read in the U.S. It inaugurated a series by him on subitism and gradualism.
A core problem in the study of early Buddhism is the relation between jhana/dhyana and insight. The Buddhist tradition has incorporated two traditions regarding the use of dhyana. There is a tradition that stresses attaining insight (bodhi, prajna, kensho) as the means to awakening and liberation. But it has also incorporated the yogic tradition, as reflected in the use of jhana, which is rejected in other sutras as not resulting in the final result of liberation. The problem was famously voiced in 1936 by Louis de La Vallee Poussin, in his text Musila et Narada: Le Chemin de Nirvana.
Schmithausen, in his often-cited article On some Aspects of Descriptions or Theories of 'Liberating Insight' and 'Enlightenment' in Early Buddhism, notes that the mention of the four noble truths as constituting "liberating insight", which is attained after mastering the Rupa Jhanas, is a later addition to texts such as Majjhima Nikaya 36. Schmithausen discerns three possible roads to liberation as described in the suttas. Vetter adds a fourth possibility, which pre-dates these three:
This problem has been elaborated by several well-known scholars, including Tilman Vetter,Johannes Bronkhorst, and Richard Gombrich.