nitya – “perpetual, constant, eternal, invariable, fixed, necessary, obligatory.”
In the context of Vratas, the term nitya refers to those vratas which are to be performed regularly and are considered to be necessary or obligatory, contrasted with those which are naimittika – “occasional” – i.e. undertaken for a specific reason or at a special time or place. Nitya karman refers to any constant action or duty (such as daily worship). No particular advantage comes from such obligatory actions, but their non-performance is associated with inauspicious effects.
Nityasiddha is sometimes used to refer to one who is “eternally perfected” or who has attained perfection from spiritual practices. Nityalila is the “eternal enjoyment” of the company of God (according to Vallabha).
In the Devi Mahatmya the goddess is desribed as follows:
She is eternal (nitya) having the universe as her form; by her is all this spread out.
Even so, her coming-into-being is manifold; hear (about this) from me.
When she becomes manifest, for the sake of accomplishing the purposes of the gods,
Then she is said to be born in the world, even though she is eternal (nitya).
nitya is often found as an element of god or goddess titles, for example: Mundamaladharanitya – “always wears a skull rosary” or Nityanandavidhayini – “always holds a severed head” (both names of Chinnamasta).
There is some evidence to suggest that in the Rg Veda nitya means “pleasing”, “beloved” or “favourite” and (that which is one’s) “own”