Zenjo (禅定)
Zenjo (dhyaana in Sanskrit, jhaana in Pali) is to meditate and observe the truth while focusing one's mind. It also means the stability attained through Zenjo under which neither one's mind nor body is unsettled by anything. It is the compound term of 'zen,' a transcription of Sanskrit dhyaana, and its free translation 'jo,' which has the same meaning as zanmai.
Summary
Originally, it was Daigo Zenna haramitsu (paramita), one of six haramitsu that bosatsu (Bodhisattva) of Mahayana Buddhism was obliged to practice. Zenna (Dhyāna) is translated as 'Zenjo' and means stabilizing the unsettled mind by thinking the truth.
In the process of attaining such a state, according to Agama Sutra, there are four stages ranging from shozen (the first Dhyana) to daishizen (the fourth Dhyana). These are collectively called 'Shijoryo' (also referred as Shizen). After that, according to the sutra, there are another four stages called Kumuhensho (the lowest heaven of Mushikikai (the realm of non-form)),Shikimuhensho (the second lowest heaven of Mushikikai), Musho-usho (the third lowest heaven of Mushikikai) and Hisohihisosho (the highest heaven of Mushikikai). These are collectively called 'Shimushikijo' (four concentrations of the formless realm) and are followed by Kushidaijo (Nine Successive Stages of Concentration) and Hyakuhachi zanmai (one hundred eight kinds of samadhi).
In Buddhism, religious precepts, Zenjo and wisdom are referred to collectively with the common phrase 'kai (precepts), jo (zenjo) and kei (wisdom).'
(Regarding the relationship between Zenjo and wisdom, refer to Shikan (Tendai meditation)).
Zenjo and Zazen (meditation)
The Zen sect, which abolished Zenjo practices and pursued Tongo (prompt enlightenment), was established in China. Although this sect brought forth a new definition of Zenjo (i.e., Zazen) and had the aspect of completely denying the existing Buddhism, it prospered in the era of the Tang Dynasty. The Zen sect was introduced into Japan by Eisai and Dogen during the period of the Southern Sung Dynasty. In Japan, Zenjo practices were revived again and became popular as the principal tokumoku (virtue) of samurai (warriors).