Shomyo Nenbutsu (称名念仏)
Shomyo Nenbutsu is one practice of praying to the Buddha (nenbutsu (Buddhist invocation)), and is a way to chant the name of the Buddha such as 'namuamidabutsu.'
Basically, 'Shomyo' (intoning or chanting the name of the Buddha) and 'Nenbutsu' are two separate words.
About Nenbutsu
Nenbutsu in Sanskrit is buddhānusmṛti, and is an original word as one of the Rokunen (the six repetitions of prayers; nenbutsu, nenbo (recollection of the dharma), nenso (to be mindful of the samgha), nenkai (mindfulness of morality), nense (mindful of charity) and nenten (desire heaven)). In this case, nenbutsu means that within the attributes of the Buddha, there is Daiji (great compassion), Daihi (great mercy) and Daikomyo (great hope). Through these limitless powers, one must believe in easing the pain of all living things and pray to become one in the same as the Buddha.
In nenbutsu, there is a method of contemplation where the mind is suspended to see things more correctly called 'gojoshinkan' (five approaches to meditation) wherein 'Nenbutsukan' (observation of mindfulness of the Buddha) is included. It is said that Nenbutsukan in this case is a way to manipulate and calm the mind against hindrances such as sleep and stress.
As seen from above, there are many ways to interpret Nenbutsu.
Within the word "Nen" there are meanings such as 'okunen' (something one always remembers), 'shinen' (thoughts), 'shinnen' (thoughts and concentration), 'kannen' (observation and contemplation) and 'shonen' (shomyo and nenbutsu; literally: "naming the Buddha"), and within the word "butsu" there is the meaning of busshin (the body of the Buddha) and butsumyo (the name of the Buddha).
When contemplating 'the Buddha' as Busshin (the body of the Buddha), such as looking at tangible images of the Buddha or Buddhist statues, or when meditating on the Buddha as Hosshin (dharma body or Buddhism's highest form of existence) appearing as an essential truth of reality, the meaning of 'Nen' naturally changes to the meaning of Butsu (Buddha). When contemplating Hosshin (dharma body), a person is 'seeing a principle' and the word "Nen" holds the meaning as it is used in okunen, shinen shinnen and so on. When looking at concrete images of the Buddha, because one is looking at the shape of a real body or statue, Nen takes on the meaning as it used in Kannen (conception). When contemplating the Buddha as Butsumyo (the name of the Buddha), since a name is a word used to call a thing, the meaning of Nen should be seen as Shonen.
As seen above, within the word Nenbutsu as well, not just one, but various meanings are attached in to it in Buddhism. It is possible to say that the spreading of the word Nenbutsu corresponds to the spreading of images of the Buddha.
In old sutras, as Buddhist disciples are said to chant 'namubutsu' as a nenbutsu for remembering the living Buddha Shakyamuni, nenbutsu is an actual prayer. Before long, this kind of nenbutsu was to consider Shakyamuni as a master, and by turning their entrance to nirvana (death) as an occasion, one image was spread, and nenbutsu for the Buddha through religious principles began to be preached. Nenbutsu in this case does not ignore Shakyamuni as a person, but dharma as a principle, rather than a Buddha that simply exists in the real world.
However, as the image of Shakyamuni gradually becomes vague among Buddhist disciples, they begin to pray to the Buddha purely as 'a principle.'
In this case, nenbutsu begins to have nearly the same meaning as 'nenbo,' but as long as it is a nenbutsu, it is different from a simple nenbo. It is a way to pray to Buddha as a hosshin (dharma body) captured within the principles of Buddhism. With the passage of time, nenbutsu came to be called a sacred act of praying to the Buddha. In going so far as to make statues or drawings of the Buddha, the statue or appearance of the Buddha is being created by kannen (observation and contemplation), and it becomes kanzo (looking at an image) or kanso (meditation or contemplation).
About Shomyo Nenbutsu
Within the spread of nenbutsu, there was a desire in believers to try to tangibly perceive the Buddha by chanting the name Nenbutsu, like 'namubutsu' in an original sutra.
What has always been connected to the practices of believers is the 'nenbutsu for Amida Buddha.'
In ancient times, nenbutsu of Amida Buddha was preached as a representative of Shobutsugenzenzanmai (various forms of the Buddha appearing in meditation) in the "Hanju Zanmai-kyo Sutra" (sutra of the meditation to behold the Buddhas), and this became the basis of jogyo zanmai (constantly walking samadhi) in the Tendai sect. In China, three popular lineages of nenbutsu are the Eon (Eastern Jin) of Byakuren-sha, nenbutsu by Shandao and nenbutsu of Jimin-ryu by Enichi. In particular is the nenbutsu by Shandao which was introduced to Japan and became as fundamental as the Jodo sect. These three lineages of nenbutsu each have their own characteristics: Nenbutsu of Byakuren-sha mainly having characteristics as a kanso nenbutsu (to contemplate Buddha (especially Amitabha) in the mind and repeat his name), nenbutsu of Shandao having characteristics as kosho nenbutsu (invocatory prayer), and nenbutsu in Jimin-ryu having the characteristics of zen.
As described, there are various forms of nenbutsu within the nenbutsu of Amida Buddha, and the prevailing account of kosho nenbutsu is by chanting the six character name of nenbutsu '南無阿弥陀仏' (namuamidabutsu). It has been preached that if a person chooses a way to recite omyogo (the name of the Buddha), they will be able to understand the teachings of Buddha even in a Jokuse (a world stained with defilements) of Mappo (latter day of Buddhism). Among the shomyo nenbutsu, within gestures and movements attempting to grasp a tangible Buddha came the appearance of odori nenbutsu (dancing nenbutsu).
For those who seek for the basic strength to the foundation of life in human lives through a pure form of nenbutsu, there is the Yuzu Nenbutsu (interpenetrated recitation) by Ryonin and further teachings by Honen and Shinran.