Wikipedia
In Theravada Buddhist phenomenology, Kalapas are defined as the smallest units of physical matter. Kalapas are described as tiny units of materiality, “tens of thousands of times smaller than a particle of dust,” coming into existence and disappearing in as little as a billionth of a second or a trillionth of the blink of an eye. Kalapas are understood by some Therevada thinkers as actual subatomic particles and the smallest units of materiality.
Kalapas are not mentioned in the earliest Buddhists texts, such as the Tripitaka, but only in the Abhidhammattha Saïgaha, an Abhidhamma commentary that was composed between the 5th and 11th centuries. They are not universally accepted in Theravada Buddhism, and the Buddha never directly speaks of kalapas.
According to the description found in the Abhidhammattha Saïgaha, Kalapas are said to be invisible under normal circumstances but visible as a result of meditative samadhi. Kalapas are composed of eight inseparable elements of material essence in varying amounts which are:
The first four elements are called primary qualities, and are predominant in kalapas. The other four are secondary properties that derive from the primaries. Certain kalapas are said to also include additional elements, including sound, sex, body, mind-base and life. In contemporary Buddhist meditation practice, the observation and analysis of kalapas is a type of vipassana practice that aims to allow direct observation of impermanence and non-self. Contemporary adherents of practices related to the observation and analysis of kalapas include U Ba Khin, S.N. Goenka and Pa Auk Sayadaw. Mahasi Sayadaw expressed a belief that kalapas played a role in aging, death and rebirth.