Upacāra-samādhi

upacāra-samādhi is translated as "access concentration," "neighborhood concentration," "threshold concentration," etc. It is one of the three stages of samādhi identified in the Pali tradition.

This stage of samadhi follows the stage of preliminary concentration (parikamma-samādhi); it arises when the five hindrances are no longer present, and is signified by the appearance of the counterpart sign (paṭibhāga nimitta).

One Teacher, Many Traditions states:

As she continues meditating, the five hindrances...are gradually suppressed by the five absorption factors (jhānaṅga). Here “suppression” means manifest hindrances do not appear. Suppression of the five hindrances leads to greater clarity and ability to discern the functioning of the mind. The mind and body are at ease; the mind is virtuous, and one feels either pleasure or equanimity.

Untroubled by manifest afflictions and the five hindrances, the mind becomes more concentrated. The counterpart sign (paṭibhāga nimitta) arises and access concentration is attained. While imperfections are seen in the learning sign, the counterpart sign is brighter and more purified. Luminous, beautiful, and vivid, it has no physical color or shape and is an object only of mental consciousness. It lacks the three characteristics of impermanence and so forth. Stabilizing the counterpart sign is difficult, so the meditator must guard it carefully and continue practice to attain full absorption.

Sources

• Dalai Lama; Thubten Chodron (2014), Buddhism: One Teacher, Many Traditions, Wisdom Publications 
• Nyanatiloka Thera (2019), Nyanaponika Thera, ed., Buddhist Dictionary: Manual of Buddhist Terms and Doctrines, Pariyatti Publishing