Sudden awakening or Sudden enlightenment (Chinese: 頓悟; pinyin: Dùnwù; Japanese pronunciation: tongo), also known as subitism, is a Buddhist idea which holds that practitioners can achieve an instantaneous insight into ultimate reality (Buddha-nature, or the nature of mind). This awakening is described as being attained "suddenly," "in one glance," "uncovered all together," "beyond conceptual elaborations” or "together, completely, simultaneously," in contrast to "successively or being uncovered one after the other." It may be posited as opposite to gradualism, an approach which says that insight can be achieved only through a long gradual step by step process.

The application of the term "subitism" 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.

The Chinese term dun 頓, as used in dunwu 頓悟, translated as "subite," sudden, has a broader meaning than "sudden." It is more apt translated as "in one glance," "uncovered all together," or "together, completely, simultaneously," in contrast to "successively or being uncovered one after the other." It means that all aspects of Buddhist practice are realized, or actualized, simultaneously, and not one after another as in a gradual or linear school curriculum. Specifically, the defilements are not erased gradually, by good works, but simultaneously.

Subitizing, also derived from the Latin adjective subitus, is the rapid, accurate, and confident judgments of numbers performed for small numbers of items.

The distinction between sudden awakening and gradual awakening (Chinese: 漸悟) has its roots in Indian Buddhism. It was first introduced in China at the beginning of the 5th century by Daosheng. The term became of central importance in Chan Buddhism, where it is used to denote the doctrinal position that awakening, the comprehension or realization of the Buddhist teachings, happens simultaneously, and is not the fruit of a gradual accretion or realisation.

In the 8th century the distinction became part of a struggle for influence at the Chinese court by Shenhui, a student of Huineng. Hereafter "sudden enlightenment" became one of the hallmarks of Chan Buddhism, though the sharp distinction was softened by subsequent generations of practitioners.

This softening is reflected in the Platform Sutra of Huineng.

While the Patriarch was living in Bao Lin Monastery, the Grand Master Shen Xiu was preaching in Yu Quan Monastery of Jing Nan. At that time the two Schools, that of Hui Neng of the South and Shen Xiu of the North, flourished side by side. As the two Schools were distinguished from each other by the names "Sudden" (the South) and "Gradual" (the North), the question which sect they should follow baffled certain Buddhist scholars (of that time). (Seeing this), the Patriarch addressed the assembly as follows:
So far as the Dharma is concerned, there can be only one School. (If a distinction exists) it exists in the fact that the founder of one school is a northern man, while the other is a southerner. While there is only one dharma, some disciples realize it more quickly than others. The reason why the names 'Sudden' and 'Gradual' are given is that some disciples are superior to others in mental dispositions. So far as the Dharma is concerned, the distinction of 'Sudden' and 'Gradual' does not exist.