Shikantaza

 is a Japanese translation of a Chinese term for "zazen" introduced by Rujing, a monk of the Caodong school of Zen Buddhism, to refer to a practice called "Silent Illumination", or "Serene Reflection", by previous Caodong masters. In Japan, it is associated with the Soto school. Unlike many other forms of meditation, shikantaza does not require focused attention on a specific object (such as the breath); instead, practitioners "just sit" in a state of conscious awareness.
Etymology.
The term "shikantaza" is attributed to Dōgen's teacher Tiantong Rujing (1162-1228), and it literally means, "nothing but sitting." In other words, Dōgen means, "doing only zazen whole-heartedly" or "single-minded sitting."
"Shikantaza" is the Sino-Japanese reading of the Chinese words "zhǐguǎn" 只管 "by all means; merely, simply; only concerned with" and "dǎzuò" 打坐 "[Buddhism/Daoism] sit in meditation". The "Digital Dictionary of Buddhism" translates "shikan" or "zhǐguǎn" 只管 as "to focus exclusively on", "taza" or "dǎzuò" 打坐 as "to squat, sit down cross-legged", which corresponds with Sanskrit "utkuṭuka-stha", and translates "shikan taza" from "zhǐguǎn dǎzuò" 只管打坐 (or "qíguǎn dǎzuò" 祇管打坐 with qí "earth god; local god") as "meditation of just sitting", explained as the "Zen form of meditation chiefly associated with the Sōtō school, which places emphasis on emptying the mind, in contrast to the kōan method".
James Ishmael Ford says some authors hypothetically trace the root of "shikantaza" "just sitting" to vipassana meditation, but "this is far from certain." Japanese has many homophones pronounced "shikan", and this etymological mix-up about "shikan" 只管 "only; just" stems from a more commonly used word that translates the Sanskrit ""śamatha" and "vipaśyanā"," names for the two basic forms of Buddhist meditation: Japanese "shikan" 止観 "concentration and observation" (as practiced by the Tendai sect), from Chinese "zhǐguān" 止觀 "[Buddhism] keep mental calm while observing the universe" (cf. the "Mohe Zhiguan"), which compounds "shi" or "zhǐ" 止 "stop; stabilize; "śamatha"" and "kan" or "guān" 觀 "observe; contemplate; "vipaśyanā". An instance of the confusion of 止観 for 只管 is Steve Hagen's claim that "shi" [Hagen is referring to Dōgen's '只'] means tranquility [= '止'], "kan" [Hagen is referring to Dōgen's '管'] means awareness [= '観'], "ta" means hitting exactly the right spot (not one atom off), and "za" means to sit."
Origins and development.
Silent illumination.
"Just sitting" is the term that Rujing used for "silent illumination" or "silent reflection" (Chinese "mòzhào" 默照; Japanese "mokushō") which may be understood as the integrated practice of shamatha (calming the mind) and vipashyana (insightful contemplation), and was the hallmark of the Chinese Caodong school of Chan. 
It is not merely just the union of calming and insight, which had already been developed within the Tiantai Buddhist tradition in medieval China. Rather, it is a description of the natural essence and function of the mind. In this sense, it can be traced back to the earliest Chan teachings of Bodhidharma.
The first Chan teacher to articulate silent illumination was the Caodong master Hongzhi Zhengjue (1091—1157), who wrote an inscription entitled "silent illumination meditation" ("Mokushō zen" 默照禅 or "Mòzhào chán" 默照禪). According to Taigen Dan Leighton, this practice is a "nondual objectless meditation" which:
...involves withdrawal from exclusive focus on a particular sensory or mental object to allow intent apprehension of all phenomena as a unified totality. This objectless meditation aims at a radical, refined nondualism that does not grasp at any of the highly subtle distinctions to which our familiar mental workings are prone and which estranges us from our experience. Such subject-object dichotomization is understood as artificial, a fabrication. 
Silent illumination is also objectless in the sense of not seeking after specific limited goals. The ultimate purpose of spiritual practice, universally awakened heart/mind, cannot be set apart from our own inherent being and our immediate, moment-to-moment awareness.
A. Charles Muller explains that this method, which involved seeking enlightened wisdom through complete stillness of the mind, became "the main practice of the Sōtō Zen school, where Dōgen characterized it by such terms of as 'just sitting' 只管打坐.".
Shikantaza's origins can also be traced back to silent illumination. However, some teachers see it as different from the teachings of Hongzhi Zhengjue in terms of practice and theory.
Dogen.
In the thirteenth century, Dōgen Zenji who introduced the Soto School in Japan, used much of Hongzhi's writings on silent illumination to help shed light on what he termed "shikantaza". From then on the practice of shikantaza has been primarily associated with the Soto school. While silent illumination is in theory a "methodless method", it is important to realize that, the Dogen practice of shikantaza "took a somewhat different approach."
The Chinese Chán Master Shengyen has stated, that shikantaza is similar to silent illumination.
Modern interpretations.
Master Shengyen explains the meaning of the term in this way:
According to Merv Fowler, shikantaza is described best as, 
Shikantaza is often termed a goalless meditation in quiet awareness, 
Practice.
Soto.
In his work "Fukan zazengi", Dogen writes of, 
Fred Reinhard Dallmayr writes, 
According to Master Shengyen, 
Sanbo Kyodan.
The modern Japanese Zen master, Haku'un Ryōko Yasutani says: 
In contrast to this opinion, some of the Zen masters in Loori's book "The Art of Just Sitting" deride Yasutani's description, giving their own version as the right or correct way to do shikantaza.
Concerning the Rinzai school, John Daido Loori writes, 
Haku'un Yasutani agrees, stating, 
Complementary practices.
In Japan, vipassana and shamatha are sometimes used in addition to shikantaza as complementary practices.