The terms “nimitta” or “sign” and patibhaga-nimitta or “counter-part sign” are frequently referred to in this essay, and it is best to clarify their meaning at the outset. The “sign” means a characteristic mark or phenomenon which accompanies and helps identify an experience. For example, the flu is often accompanied by weakness and nausea; here nausea would be a sign of the flu. Extreme joy may be accompanied by a feeling of lightness of body and tears; these would be signs of joy. A doctor looks for certain signs which characteristically accompany specific illnesses.
In the same way, certain signs are characteristic of entering deep states of right concentration and are intrinsic to the jhanic state. According to the definitions (taken from Commentarial sources) found in Nyanatiloka’s Buddhist Dictionary, there are three types of nimitta. The first type is the parikamma-nimitta, which refers to the perception of the object at the very beginning of concentration - it is also known as the “preparatory image or sign.” When the mind reaches a weak degree of concentration, a still unsteady and unclear image or sign called the “acquired sign” (uggaha-nimitta) arises. This perception precedes the appearance of an entirely clear and static image called the “counter-image” or “counter-sign” (the patibhaga-nimitta). The appearance of this third type of nimitta signals the appearance of neighbourhood (or access) concentration, the state that precedes full jhanic absorption.
Both of these states share the same sign but differ only in the intensity of the component (state) factors. As mentioned in this definition, the counter-part sign is understood as a more refined and clarified version of the sign and is the natural result of heightened awareness and concentration. By knowing these signs, both the student and teacher are helped to assess the success or failure of the corresponding concentration attainments.
This reflection by Ajahn Sona is from, The Mystery of the Breath Nimitta.