That which is beyond any units of measurement or that which is transcendental, beyond the mundane in Hinduism is known as Amatra. It refers to Brahman and Omkara. Upanishads conceive Brahman not as an object beyond senses and the mind but the Brahman that is cognized in three states of consciousness – wakefulness, dream state, and state of deep sleep (jagrata, swapna and sushupti) as an embodied being.
Since it is incomparable – it is unlike everything we know – the closest representation of Brahman can only be pranava which has three states – a, u and m – comparable to the three embodies states – Brahman, pranava and amatra.
At the microcosmic level, the individual has three states – wakefulness, dream state, and state of deep sleep, and he is known as Vishva, taijasa and prajna in these three states respectively.
At the macrocosmic level, the three states are manifested universe, subtle existence and the un-manifested universe. The God who permeates these three levels of creation, is known as Virata, Hiranyagarbha and Ishvara respectively, though the same God is known by different names due to differentiating adjuncts.
If the adjuncts are not there, there will not be any differentiating mark. What remains is the only undifferentiated universal, all pervasive consciousness. Since it is different from these differentiating adjuncts and above them, Brahman in called amatra. Meditation on Brahman through Omkara is considered the best and unfailing. Tasya vacakah, pranavah, tajjapah tadartha bhavanam – states Sage Patanjali. Om too is referred as amatra.