Ritambhara is a state of purified intellect. In Samkhya yoga psychology, citta or antahkarana (the internal organ) is said to be composed of three segments, respectively called, manas, ahankar, and buddhi (Samkhya karika – 33). The internal organ is, since birth, full of two types of impurities, namely,
Kleshas (the five afflictions) and
Karma Samskaras (traces of past experiences and deeds). These two impurities restrict the capacity of prajna (intellect) to have knowledge. So the natural intellect of all of us is called samanya prajna (ordinary intellect). It is influenced by Agama or Shruta (what has been taught to us), and anumana (what we infer). It yields incomplete and defective knowledge.
Patanjali has recommended ashtanga yoga (an eight-fold path of practice) to remove the impurities of the intellect, resulting into the brightening of the clarity of knowledge. The highest stage of this effect is arousal of viveka-khyati (self – knowledge) (Yoga Sutra – II: 28). This process begins with the citta becoming free from Krishna-vritti (its modifications), thereby having the capacity of uniting perfectly (without any movement) with any object of contemplation. The increasingly finer state of samapatti is also called sabija-samadhi. The finest among these states is called “nirvicara-samapatti.” In it the adept’s mind is filled with undisturbed inner calm (adhyatma-prasada) (Yoga Sutra – I: 47). Then the intellect comes upon the failing truth. It is called ‘ritambhara’, that is, “that which gains unfailing truth.” (Yoga Sutra – I: 48). It yields pure knowledge, which makes for “nirbija-samadhi’ (Yoga Sutra – I: 51)
Source - Encyclopedia of Hinduism Volume III page 595-96 - published by IHRF.