Nouns

(n)constant, constant quantity, invariablea quantity that does not vary
(n)constanta number representing a quantity assumed to have a fixed value in a specified mathematical context“the velocity of light is a constant”

Adjectives

(s)changeless, constant, invariant, unvaryingunvarying in nature“maintained a constant temperature”, “principles of unvarying validity”
(a)constantsteadfast in purpose or devotion or affection“a man constant in adherence to his ideals”, “a constant lover”, “constant as the northern star”
(s)ceaseless, constant, incessant, never-ending, perpetual, unceasing, unremittinguninterrupted in time and indefinitely long continuing“the ceaseless thunder of surf”, “in constant pain”, “night and day we live with the incessant noise of the city”, “the never-ending search for happiness”, “the perpetual struggle to maintain standards in a democracy”, “man's unceasing warfare with drought and isolation”, “unremitting demands of hunger”