Samatha and vipassanā are the representative technical terms of the Buddhist practice. They are the core technical terms that illustrate most clearly the practice system of Theravada Buddhism. Early on, these two technical terms were translated as 止 [concentration] and 觀 [seeing] in Chinese.
The important literary works of Bhikkhu Yeongga Hyeongak [永嘉玄覺], a direct disciple of the Sixth Patriarch Venerable Huineng [慧能] and also the famous author of Jeungdo Ga [Song of Actualizing the Way], include Seonjong Yeongga Jip [禪宗永嘉集]. This book has ten chapters in all. The core of the practice section is three chapters: chapter four, titled Samatha; chapter five, titled Vipassanā; and chapter six, titled Upekkhā.
Therefore, we know that samatha and vipassanā were deeply understood in China early on. It has been taught that these practices—concentration [samatha] and insight [vipassanā]—should be done in tandem. This practice has been translated as 止觀兼修 [practicing samatha and vipassanā together]. In Seon Jong [Chan Jong or Zen], it has come to be understood as 定慧雙修 [practicing samatha and vipassanā together] through the generations.
Then, this begs the question, did the Buddha personally expounded on samatha and vipassanā in the Early Buddhist discourses? How did the Buddha define samatha and vipassanā?
The Buddha clearly defines samatha and vipassanā in many of the Early Buddhist discourses. In the Fools Discourse (A2:3:11) of the Aṅguttara Nikāya the Buddha definitely connects samatha with concentration [samadhi] and vipassanā with insight [paññā]. He clarifies that concentration is a practice one uses to overcome desire and insight is a practice one uses to overcome ignorance.
The Concentration Discourse of the Aṅguttara Nikāya supplies clear answers regarding samatha and vipassanā. The expressions of ‘internal serenity of mind’ and ‘higher wisdom of insight,’ are seen in these three discourses (A4:92-94). Samatha is a practice of developing serenity or having serenity. Vipassanā is a practice of developing insight or having insight.
The Concentration Discourse explains that in order to obtain samatha, one can approach another who gained internal serenity of mind and inquire, “How, friend, should the mind be steadied? How should the mind be composed? How should the mind be unified? How should the mind be concentrated?” (A4:94)
The Buddha explains that in order to obtain vipassanā, one can approach another who gained the higher wisdom of insight into conditioned phenomena and inquire, “How, friend, should conditioned phenomena be seen? How should conditioned phenomena be explored? How should conditioned phenomena be discerned by insight?” (A4:94)
The Buddha had clearly shown samatha is a practice to develop concentration by fixing the mind on an object and achieving serenity; vipassanā is a practice to contemplate and observe conditioned phenomena [saṇkhāra] allowing one to clearly see
the impermance.suffering.non-self of conditioned phenomena.
The commentary literature describes samatha and vipassanā separately as mundane samatha [lokiya-samatha], supramundane samatha [lokuttara-samatha], mundane vipassanā [lokiya-vipassanā], and supramundane vipassanā [lokuttara-vipassanā]. (SA.iii.21; Pm.15; MAT.i.238, etc.) Concentration and insight are also described separately as mundane concentration, supramundane concentration, mundane insight, and supramundane insight. (DA.ii.425; UdA.69, etc.) The supramundane [lokuttara] takes Nibbāna as its meditation object (Vis.XIV.15, etc.). The mundane [lokiya] does not take Nibbāna as its meditation object.
For those who have not experienced the path and the fruit, I would like to clarify the fact that samatha and vipassanā discussed in this entire book is about the practice of mundane samatha and mundane vipassanā—the practice to experience the path and the fruit and to realize Nibbāna. The supramundane samatha and vipassanā are not included here. The mundane samatha and vipassanā—the practice of experiencing the path and fruit and realizing Nibbāna—are meaningful and relevant to us. Also, the explanations including the supramundane can make samatha and vipassanā a controversial debate at the slightest slip of interpretation.
“In the Early Buddhist discourses, the Buddha clearly defined samatha and vipassanā. Samatha is a practice of developing concentration to bring serenity of mind. Vipassanā is a practice of exploring conditioned phenomena to clearly see the impermanence·suffering·non-self with insight.”
[Introduction to Early Buddhism authored by Bhikkhu Kakmuk and translated by Nancy Acord]
 Upekkhā means equanimity.
 Seon Jong is a representive order of Korean Buddhism.