Nididhyasana in Hinduism is a stage in meditation. The verbal root ‘dhyai) (dhyayati) means to observe, to see, to meditate, to remember, etc. To intensify this meaning one may add the prefix ‘ni’ to this root – nidhyai (nidhyayati). This would then mean to meditate profoundly, to meditate repeatedly, to observe carefully, etc. Nidhyana, nididhyasa, and nididhyasana are nouns derived from this verb, and they mean profound meditation, repeated meditation, intense observation, correct seeing, etc (nitaram dhyana). Shankara, commenting on this word, says – Supreme Self is to be meditated upon with firmness and with a steady mind” (niscayena dhyatavya) (Brihadaranyaka Upanishad II.4.5 and IV.5.6).
Mukti Upanishad also mentions these three steps for the direct vision of Supreme Self, namely, hearing, reflection and prolonged meditation (sravana-manana-nididhyasanani).
The last three steps of the ashtanga yoga of Patanjali are the steady and constant fixing of the mind on the object of meditation (dharana), continous and unbroken meditation (dhyana) and total absorption and concentration (Samadhi). These three limbs of Yoga are collectively called the internal means of Yoga (antaranga sadhana).
Nididhyasana might be considered as a combination of all these three. Samadhi is the last stage of the yogic journey. It is the supreme form of meditation. Patanjali speaks about two types of Samadhi, namely, conscious absorption (samprajnata samadhi) and supra conscious Samadhi (asamprajnata Samadhi). In the conscious Samadhi, the consciousness of the object of meditation and of individuality do not disappear. There is some kind of subject-object distinction. But in the supra-conscious Samadhi, this barrier too is broken. The one who meditates and the object of meditation are fused together; there is no longer any consciousness of the object. It is the highest form of Yoga. A yogi cannot remain in it for long; his material body will break down and he will pass over to the state of kaivalya. In Vedanta, too, nididhyasana is the highest form of meditation, and this will lead the seeker to direct vision (darshana) of the Supreme Self and eventually merger into his Self.