The Buddha explained that concentration is ‘one-pointedness of mind [cittassa ekaggataa]’ in many discourses. The commentaries explained this one-pointedness as an object (PsA.230) and the object of mind nearing concentration as a counterpart sign [paṭibhāga-nimitta].
Samatha is a state in which the mind is focused on an object called a sign. According to all the commentary literature including the Path of Purification, such concentration is developed through access-concentration [upacāra-samādhi] and absorption-concentration [appanā-samādhi]. (Vis.IV.31-33) The Comprehensive Manual of the Abhidhamma explains the concentration practice in further detail as preparatory, access, and absorption concentration.
First, the preparatory practice is the elementary practice done prior to the arising of access-concentration. Specifically, the preparatory practice is the state up to or immediately prior to the arising of a counterpart sign while the five hindrances are suppressed. Access-concentration occurs when the five hindrances are suppressed and from the time a counterpart sign appears, immediately prior to the arising of the mind of gotrabhū, while entering the state of jhāna. The absorption-concentration is the mind that arises right after the mind of gotrabhū; it means the states of the first jhāna through the fourth jhāna.
Unlike this, vipassanā is a practice that enables one to see clearly the momentary arising and passing away of dhamma and is not a practice in which one focuses on a sign; therefore, a counterpart sign does not arise. Since a counterpart sign does not arise, there is no absorption-concentration in the vipassanā practice. Of course, without a high level of concentration, it is impossible to see an object thoroughly as impermanent, suffering, and non-self. Therefore, the high level of concentration during the vipassanā practice cannot be called absorption-concentration. The commentary literature such as the Path of Purification calls the high level of concentration that occurs during the vipassanā practice momentary-concentration [khaṇika-samādhi].
The Path of Purification calls the practitioner who does not practice samatha first but practices vipassanā right away, a ‘dry vipassanā practitioner’ or a ‘pure vipassanā practitioner.’ Momentary-concentration is a highly-focused state that occurs during this pure vipassanā practice; it is said to be equal to access-concentration or comparable to the first jhāna of the samatha practice. Thus, there is a basis for concentration in the vipassanā practice.
“Concentration is the mind being focused on one-pointedness. The concentration achieved through samatha is called access-concentration or absorption-concentration; the high-level concentration of vipassanā is called momentary-concentration.”
[Introduction to Early Buddhism authored by Bhikkhu Kakmuk and translated by Nancy Acord]
 The gotrabhū is one who has entered the lineage of the Noble Ones.